Prospectus Supplement
(To Prospectus dated June 29, 2018)

Calamos Global Total Return Fund
Up to 2,685,638 Common Shares

Calamos Global Total Return Fund (the “Fund,” “we,” “us,” or “our”) has entered into a sales agreement, dated June 29, 2018 (the “sales agreement”) with JonesTrading Institutional Services LLC (“JonesTrading”) relating to the Fund’s common shares of beneficial interest (“common shares”) offered by this prospectus supplement and the accompanying prospectus. In accordance with the terms of the sales agreement, we may offer and sell up to 2,685,638 of our common shares, no par value per share, from time to time through JonesTrading as our agent for the offer and sale of the common shares. As of April 30, 2018, the Fund had offered and sold 314,362 common shares pursuant to a prior agreement with JonesTrading. Under the Investment Company Act of 1940, as amended (the “1940 Act”), the Fund may not sell any common shares at a price below the current net asset value of such common shares, exclusive of any distributing commission or discount. The Fund is a diversified, closed-end management investment company that commenced investment operations in October 2005. Our investment objective is to provide total return through a combination of capital appreciation and current income.

Our common shares are listed on the NASDAQ Global Select Market (“NASDAQ”) under the symbol “CGO.” As of April 30, 2018, the last reported sale price for our common shares was $13.83 per share. As of April 30, 2018, the last reported net asset value for our common shares was $13.05.

Sales of our common shares, if any, under this prospectus supplement and the accompanying prospectus may be made in negotiated transactions or transactions that are deemed to be “at the market” as defined in Rule 415 under the Securities Act of 1933, as amended (the “1933 Act”), including sales made directly on the NASDAQ or sales made to or through a market maker other than on an exchange.

JonesTrading will be entitled to compensation of 100 to 250 basis points of the gross sales price per share for any common shares sold under the sales agreement, with the exact amount of such compensation to be mutually agreed upon by the Fund and JonesTrading from time to time. In connection with the sale of the common shares on our behalf, JonesTrading may be deemed to be an “underwriter” within the meaning of the 1933 Act and the compensation of JonesTrading may be deemed to be underwriting commissions or discounts.

Investing in our securities involves certain risks, including the Fund’s use of leverage. You could lose some or all of your investment. See “Risk Factors” beginning on page 41 of the accompanying prospectus. You should consider carefully these risks together with all of the other information contained in this prospectus supplement and the accompanying prospectus before making a decision to purchase our securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Prospectus Supplement dated June 29, 2018

This prospectus supplement, together with the accompanying prospectus, sets forth concisely the information that you should know before investing. You should read the accompanying prospectus and prospectus supplement, which contain important information, before deciding whether to invest in our securities. You should retain the accompanying prospectus and prospectus supplement for future reference. A statement of additional information, dated June 29, 2018, as supplemented from time to time, containing additional information, has been filed with the Securities and Exchange Commission (“Commission”) and is incorporated by reference in its entirety into this prospectus supplement and the accompanying prospectus. This prospectus supplement, the accompanying prospectus and the statement of additional information are part of a “shelf” registration statement that we filed with the Commission. This prospectus supplement describes the specific details regarding this offering, including the method of distribution. If information in this prospectus supplement is inconsistent with the accompanying prospectus or the statement of additional information, you should rely on this prospectus supplement. You may request a free copy of the statement of additional information, the table of contents of which is on page 82 of the accompanying prospectus, request a free copy of our annual and semi-annual reports, request other information or make shareholder inquiries, by calling toll-free 1-800-582-6959 or by writing to the Fund at 2020 Calamos Court, Naperville, Illinois 60563. The Fund’s annual and semi-annual reports also are available on our website, free of charge, at www.calamos.com, which also provides a link to the Commission’s website, as described below, where the Fund’s statement of additional information can be obtained. Information included on our website does not form part of this prospectus supplement or the accompanying prospectus. You can review and copy documents we have filed at the Commission’s Public Reference Room in Washington, D.C. Call 1-202-551-8090 for information. The Commission charges a fee for copies. You can get the same information free from the Commission’s website (http://www.sec.gov). You may also e-mail requests for these documents to publicinfo@sec.gov or make a request in writing to the Commission’s Public Reference Section, Washington, D.C. 20549-1520.

Our securities do not represent a deposit or obligation of, and are not guaranteed or endorsed by, any bank or other insured depository institution and are not federally insured by the Federal Deposit Insurance Corporation, the Federal Reserve Board or any other government agency.

TABLE OF CONTENTS

Prospectus Supplement

Prospectus

You should rely only on the information contained or incorporated by reference in this prospectus supplement and the accompanying prospectus in making your investment decisions. We have not authorized any other person to provide you with different or inconsistent information. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement and the accompanying prospectus do not constitute an offer to sell or solicitation of an offer to buy any securities in any jurisdiction where the offer or sale is not permitted. The information appearing in this prospectus supplement and in the accompanying prospectus is accurate only as of the dates on their covers. Our business, financial condition and prospects may have changed since such dates. We will advise investors of any material changes to the extent required by applicable law.

 CAUTIONARY NOTICE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the statement of additional information contain “forward-looking statements.” Forward-looking statements can be identified by the words “may,” “will,” “intend,” “expect,” “estimate,” “continue,” “plan,” “anticipate,” and similar terms and the negative of such terms. By their nature, all forward-looking statements involve risks and uncertainties, and actual results could differ materially from those contemplated by the forward-looking statements. Several factors that could materially affect our actual results are the performance of the portfolio of securities we hold, the price at which our shares will trade in the public markets and other factors discussed in our periodic filings with the Commission. Currently known risk factors that could cause actual results to differ materially from our expectations include, but are not limited to, the factors described in the “Risk Factors” section of the accompanying prospectus. We urge you to review carefully that section for a more detailed discussion of the risks of an investment in our securities.

Although we believe that the expectations expressed in our forward-looking statements are reasonable, actual results could differ materially from those projected or assumed in our forward-looking statements. Our future financial condition and results of operations, as well as any forward-looking statements, are subject to change and are subject to inherent risks and uncertainties, such as those disclosed in the “Risk Factors” section of the accompanying prospectus. All forward-looking statements contained or incorporated by reference in this prospectus supplement or the accompanying prospectus are made as of the date of this prospectus supplement or the accompanying prospectus, as the case may be. Except for our ongoing obligations under the federal securities laws, we do not intend, and we undertake no obligation, to update any forward-looking statement.

 PROSPECTUS SUPPLEMENT SUMMARY

The following summary contains basic information about us and our securities. It is not complete and may not contain all of the information you may want to consider. You should review the more detailed information contained in this prospectus supplement and in the accompanying prospectus and in the statement of additional information, especially the information set forth under the heading “Risk Factors” beginning on page 41 of the accompanying prospectus.

The Fund

The Fund is a diversified, closed-end management investment company with $166 million of total managed assets as of April 30, 2018. We commenced operations in October 2005 following our initial public offering. Our investment objective is to provide total return through a combination of capital appreciation and current income.

Investment Adviser

Calamos Advisors LLC (the “Adviser” or “Calamos”) serves as our investment adviser. Calamos is responsible on a day-to-day basis for investment of the Fund’s portfolio in accordance with its investment objective and policies. Calamos makes all investment decisions for the Fund and places purchase and sale orders for the Fund’s portfolio securities. As of April 30, 2018, Calamos managed approximately $21.8 billion in assets of individuals and institutions. Calamos is a wholly-owned subsidiary of Calamos Investments LLC and an indirect subsidiary of Calamos Asset Management, Inc.

The Fund pays Calamos an annual fee, payable monthly, for its investment management services equal to 1.00% of the Fund’s average weekly managed assets. “Managed assets” means the total assets of the Fund (including any assets attributable to any leverage that may be outstanding) minus the sum of liabilities (other than debt representing financial leverage). See “Management of the Fund” on page 51 of the accompanying prospectus.

The principal business address of the Adviser is 2020 Calamos Court, Naperville, Illinois 60563.

The Offering

The Fund and Calamos entered into the Sales Agreement with JonesTrading Institutional Services LLC (“JonesTrading”) relating to the common shares offered by this prospectus supplement and the accompanying prospectus. In accordance with the terms of the Sales Agreement, we may offer and sell up to 2,685,638 of our common shares, no par value per share, from time to time through JonesTrading as our agent for the offer and sale of the common shares. As of April 30, 2018, the Fund had offered and sold 314,362 common shares pursuant to a prior sales agreement with JonesTrading, resulting in proceeds (net of all fees, expenses and commissions) of $4,622,875. The prior sales agreement with JonesTrading has been terminated.

Our common shares are listed on the NASDAQ Global Select Market (“NASDAQ”) under the symbol “CGO.” As of April 30, 2018, the last reported sale price for our common shares was $13.83.

Sales of our common shares, if any, under this prospectus supplement and the accompanying prospectus may be made in negotiated transactions or transactions that are deemed to be “at the market” as defined in Rule 415 under the 1933 Act, including sales made directly on the NASDAQ or sales made to or through a market maker other than on an exchange. See “Plan of Distribution” in this prospectus supplement. Our common shares may not be sold through agents, underwriters or dealers without delivery or deemed delivery of a prospectus and a prospectus supplement describing the method and terms of the offering of our securities. Under the 1940 Act, the Fund may not sell any common shares at a price below the current net asset value of such common shares, exclusive of any distributing commission or discount.

Use of Proceeds

Unless otherwise specified in this prospectus supplement, we currently intend to use the net proceeds from the sale of our common shares in this offering primarily to invest in accordance with our investment objective and policies (as described under “Investment Objective and Principal Investment Strategies,” beginning on page 24 of the accompanying prospectus) within approximately three months of receipt of such proceeds. We may also use proceeds from the sale of our securities (i) to retire all or a portion of any short-term debt we incur in pursuit of our investment objective and policies, (ii) to redeem any outstanding senior securities, and (iii) for working capital purposes, including the payment of interest and operating expenses, although there is currently no intent to issue securities primarily for these purposes.

CAPITALIZATION

Pursuant to the Sales Agreement with JonesTrading, we may offer and sell up to 2,685,638 of our common shares, no par value per share, from time to time through JonesTrading as our agent for the offer and sale of the common shares under this prospectus supplement and the accompanying prospectus. There is no guaranty that there will be any sales of our common shares pursuant to this prospectus supplement and the accompanying prospectus. The table below shows our historical capitalization as of April 30, 2018 and the estimated capitalization of the Fund assuming the sale of all 2,685,638 common shares that are subject to the sales agreement on a pro forma, as adjusted basis as of April 30, 2018. Actual sales, if any, of our common shares, and the actual application of the proceeds thereof, under this prospectus supplement and the accompanying prospectus may be different than as set forth in the table below. In addition, the price per share of any such sale may be greater or less than $13.83 depending on the market price of our common shares at the time of any such sale. To the extent that the market price per share of our common shares on any given day is less than the net asset value per share on such day, we will instruct JonesTrading not to make any sales on such day.

The following table sets forth our capitalization:

•on a historical basis as of April 30, 2018; and

•on a pro forma as adjusted basis to reflect (1) the assumed sale of 2,685,638 of our common shares at $13.83 per share (the last reported sale price of our common shares on NASDAQ on April 30, 2018) in an offering under this prospectus supplement and the accompanying prospectus, and (2) the investment of net proceeds assumed from such offering in accordance with our investment objective and policies, after deducting the assumed aggregate commission of $928,559 (representing an estimated commission paid to JonesTrading of 2.50% of the gross sales price per share in connection with the sale of common shares effected by JonesTrading in each offering). 

(1)Figures do not reflect additional structural leverage related to certain securities lending programs, which were $18 million as of April 30, 2018.

SUMMARY OF FUND EXPENSES

The following table and example contain information about the costs and expenses that common shareholders will bear directly or indirectly. In accordance with Commission requirements, the table below shows our expenses, including interest payments on borrowed funds and preferred stock dividend payments, as a percentage of our average net assets as of April 30, 2018, and not as a percentage of gross assets or managed assets.

By showing expenses as a percentage of average net assets, expenses are not expressed as a percentage of all of the assets we invest. The table and example are based on our capital structure as of April 30, 2018. As of April 30, 2018, the Fund had $25 million in borrowings outstanding, $12 million in outstanding preferred shares and additional structural leverage of $18 million, collectively representing 25.9% of managed assets.

Example:

The following example illustrates the expenses that common shareholders would pay on a $1,000 investment in common shares (including an assumed total sales load or commission of 2.50%), assuming (1) total annual expenses of 2.80% of net assets attributable to common shareholders; (2) a 5% annual return; and (3) all distributions are reinvested at net asset value:

The example should not be considered a representation of future expenses. Actual expenses may be greater or less than those assumed. Moreover, our actual rate of return may be greater or less than the hypothetical 5% return shown in the example.

(1)Represents the estimated commission with respect to our common shares being sold in this offering, which we will pay to JonesTrading in connection with sales of common shares effected by JonesTrading in this offering. While JonesTrading is entitled to a commission of 1.0% to 2.5% of the gross sales price for common shares sold, with the exact amount to be agreed upon by the parties, we have assumed, for purposes of this offering, that JonesTrading will receive a commission of 2.50% of such gross sales price. This is the only sales load to be paid in connection with this offering. There is no guaranty that there will be any sales of our common shares pursuant to this prospectus supplement and the accompanying prospectus. Actual sales of our common shares under this prospectus supplement and the accompanying prospectus, if any, may be less than as set forth in the table. In addition, the price per share of any such sale may be greater or less than the price set forth in the table, depending on the market price of our common shares at the time of any such sale.

(2)Shareholders will pay a $15.00 transaction fee plus a $0.02 per share brokerage charge if they direct the Plan Agent to sell common shares held in a Plan account. In addition, each participant will pay a pro rata share of brokerage commissions incurred with respect to the Plan Agent’s open-market purchases in connection with the reinvestment of dividends or distributions. If a participant elects to have the Plan Agent sell part or all of his or her common shares and remit the proceeds, such participant will be charged his or her pro rata share of brokerage commissions on the shares sold. See “Dividends and Distributions on Common Shares; Automatic Dividend Reinvestment Plan” on page 65 of the accompanying prospectus.

(3)The Fund pays Calamos an annual management fee, payable monthly, for its investment management services in an amount equal to 1.00% of the Fund’s average weekly managed assets. In accordance with the requirements of the Commission, the table above shows the Fund’s management fee as a percentage of average net assets attributable to common shareholders. By showing the management fee as a percentage of net assets, the management fee is not expressed as a percentage of all of the assets the Fund intends to invest. For purposes of the table, the management fee has been converted to 1.46% of the Fund’s average weekly net assets as of April 30, 2018 by dividing the total dollar amount of the management fee by the Fund’s average weekly net assets (managed assets less outstanding leverage).

(4)Reflects interest expense paid on $25.9 million in average borrowings under the Fund’s Amended and Restated Liquidity Agreement with State Street Bank and Trust Company, plus $14.6 million in additional average structural leverage related to certain securities lending programs, as described in the accompanying prospectus under “Leverage.”

(5)Reflects estimated dividend expense on $12.0 million aggregate liquidation preference of mandatory redeemable preferred shares (“MRP Shares”) outstanding. See “Prospectus Summary — Use of Leverage by the Fund” and “Leverage” in the accompanying prospectus for additional information.

(6)“Other Expenses” are based on estimated amounts for the current fiscal year.

(7)The example includes sales load and estimated offering costs.

The purpose of the table and the example above is to help investors understand the fees and expenses that they, as common shareholders, would bear directly or indirectly. For additional information with respect to our expenses, see “Management of the Fund” on page 51 of the accompanying prospectus.

MARKET AND NET ASSET VALUE INFORMATION

Our common shares are listed on the NASDAQ Global Select Market (“NASDAQ”) under the symbol “CGO.” Our common shares commenced trading on the New York Stock Exchange (“NYSE”) in October 2005. On July 2, 2012, the common shares ceased trading on the NYSE and commenced trading on the NASDAQ.

Our common shares have traded both at a premium and a discount to net asset value or NAV. We cannot predict whether our shares will trade in the future at a premium or discount to NAV. The provisions of the 1940 Act generally require that the public offering price of common shares (less any underwriting commissions and discounts) must equal or exceed the NAV per share of a company’s common stock (calculated within 48 hours of pricing). Our issuance of common shares may have an adverse effect on prices in the secondary market for our common shares by increasing the number of common shares available, which may put downward pressure on the market price for our common shares. Shares of common stock of closed-end investment companies frequently trade at a discount from NAV. See “Risk Factors — Additional Risks to Common Shareholders — Market Discount Risk” on page 49 of the accompanying prospectus.

The following table sets forth for each of the periods indicated the high and low closing market prices for our common shares on the NASDAQ, the NAV per share and the premium or discount to NAV per share at which our common shares were trading. NAV is shown for the last business day of each quarter. See “Net Asset Value” on page 64 of the accompanying prospectus for information as to the determination of our NAV.

Source: Bloomberg Financial and Fund Accounting Records.

(1)Based on high and low closing market price per share during the respective quarter and does not reflect commissions.

(2)Based on the NAV calculated on the close of business on the last business day of each calendar quarter.

(3)Premium and discount information is shown for the days when the Fund experienced its high and low closing market prices, respectively, per share during the respective quarter.

The last reported sale price, NAV per common share and percentage discount to NAV per common share on April 30, 2018 were $13.83, $13.05 and 6.0% respectively. As of April 30, 2018, we had 8,489,501 common shares outstanding and managed assets of approximately $166 million.

The following table provides information about our outstanding securities as of April 30, 2018:

The following table sets forth information regarding the Fund’s outstanding bank loans, auction rate preferred shares of beneficial interest (“ARPS”) and MRP Shares as of the end of each of the Fund’s last ten fiscal years, as applicable. The information in the table shown below comes from the Fund’s financial statements for the fiscal year ended October 31, 2017, and each of the prior nine years then ended, all of which have been audited by Deloitte & Touche LLP, the Fund’s independent registered public accounting firm.

(a)Calculated by subtracting the Fund’s total liabilities (not including notes payable and MRPS) from the Fund’s total assets and dividing this by the amount of note payable outstanding, and by multiplying the result by 1,000.

(b)Calculated by subtracting the Fund’s total liabilities (not including MRPS) from the Fund’s total assets and dividing this by the number of MRPS outstanding, and by multiplying the result by 25.

(c)“Liquidating Preference per Preferred Share” means the amount to which a holder of preferred shares would be entitled upon the liquidation of the Fund in preference to common shareholders, expressed as a dollar amount per preferred share.

(d)The MRPS are not listed on any exchange or automated quotation system. The MRPS are considered debt of the issuer; and the liquidation preference approximates fair value.

USE OF PROCEEDS

Sales of our common shares, if any, under this prospectus supplement and the accompanying prospectus may be made in negotiated transactions or transactions that are deemed to be “at the market” as defined in Rule 415 under the 1933 Act, including sales made directly on the NASDAQ or sales made to or through a market maker other than on an exchange. There is no guaranty that there will be any sales of our common shares pursuant to this prospectus supplement and the accompanying prospectus. Actual sales, if any, of our common shares under this prospectus supplement and the accompanying prospectus may be less than as set forth below in this paragraph. In addition, the price per share of any such sale may be greater or less than the price set forth below in this paragraph, depending on the market price of our common shares at the time of any such sale. As a result, the actual net proceeds we receive may be more or less than the amount of net proceeds estimated in this prospectus supplement. Assuming the sale of the 2,685,638 common shares remaining under the sales agreement at the last reported sale price of $13.83 per share for our common shares on the NASDAQ as of April 30, 2018, we estimate that the net proceeds of this offering will be approximately $36,213,814 after deducting the estimated JonesTrading commissions. The estimated net proceeds do not take into account any actual sales that may have occurred between the execution of the sales agreement with JonesTrading and the date of this prospectus supplement. Unless otherwise specified in this prospectus supplement, we currently intend to use the net proceeds from the sale of our common shares in this offering primarily to invest in accordance with our investment objective and policies (as described under “Investment Objective and Principal Investment Strategies,” beginning on page 24 of the accompanying prospectus) within approximately three months of receipt of such proceeds. We may also use proceeds from the sale of our securities to retire all or a portion of any short-term debt and for working capital purposes, including the payment of interest and operating expenses, although there is currently no intent to issue securities primarily for this purpose. Pending such use of proceeds, we anticipate that we will invest the proceeds in securities issued by the U.S. government or its agencies or instrumentalities or in high quality, short-term or long-term debt obligations.

PLAN OF DISTRIBUTION

Under the sales agreement among the Fund, Calamos and JonesTrading, upon written instructions from the Fund, JonesTrading will use its commercially reasonable efforts consistent with its sales and trading practices, to sell, as our sales agent, the common shares under the terms and subject to the conditions set forth in the sales agreement. JonesTrading’s sales efforts will continue until we instruct JonesTrading to suspend sales. We will instruct JonesTrading as to the amount of common shares to be sold by JonesTrading. We may instruct JonesTrading not to sell common shares if the sales cannot be effected at or above the price designated by the Fund in any instruction. We or JonesTrading may suspend the offering of common shares upon proper notice and subject to other conditions.

JonesTrading will provide written confirmation to the Fund not later than the opening of the trading day on the NASDAQ following the trading day on which common shares are sold under the sales agreement. Each confirmation will include the number of shares sold on the preceding day, the net proceeds to us and the compensation payable by the Fund to JonesTrading in connection with the sales.

We will pay JonesTrading commissions for its services in acting as agent in the sale of common shares. JonesTrading will be entitled to compensation of 100 to 250 basis points of the gross sales price per share of any common shares sold under the sales agreement, with the exact amount of such compensation to be mutually agreed upon by the Fund and JonesTrading from time to time.

There is no guaranty that there will be any sales of our common shares pursuant to this prospectus supplement and the accompanying prospectus. Actual sales, if any, of our common shares under this prospectus supplement and the accompanying prospectus may be less than as set forth in this paragraph. In addition, the price per share of any such sale may be greater or less than the price set forth in this paragraph, depending on the market price of our common shares at the time of any such sale. Assuming 2,685,638 of our common shares offered hereby are sold at a market price of $13.83 per share (the last reported sale price for our common shares on the NASDAQ on April 30, 2018), we estimate that the total cost for the offering, excluding compensation payable to JonesTrading under the terms of the sales agreement, would be approximately $227,500.

Settlement for sales of common shares will occur on the second trading day following the date on which such sales are made, or on some other date that is agreed upon by the Fund and JonesTrading in connection with a particular transaction, in return for payment of the net proceeds to the Fund. There is no arrangement for funds to be received in an escrow, trust or similar arrangement.

In connection with the sale of the common shares on our behalf, JonesTrading may, and will with respect to sales effected in an “at the market” offering, be deemed to be an “underwriter” within the meaning of the 1933 Act, and the compensation of JonesTrading may be deemed to be underwriting commissions or discounts. We have agreed to provide indemnification and contribution to JonesTrading against certain civil liabilities, including liabilities under the 1933 Act. We have also agreed to reimburse JonesTrading for other specified expenses, including one-half of the fees and expenses of JonesTrading’s legal counsel.

The offering of our common shares pursuant to the sales agreement will terminate upon the earlier of (1) the sale of all common shares subject the sales agreement or (2) termination of the sales agreement. The sales agreement may be terminated by us in our sole discretion at any time by giving notice to JonesTrading. In addition, JonesTrading may terminate the sales agreement under the circumstances specified in the sales agreement and in its sole discretion at any time following a period of 12 months from the date of the sales agreement by giving notice to us.

The principal business address of JonesTrading is 757 Third Avenue, 23rd Floor, New York, New York 10017.

LEGAL MATTERS

Ropes & Gray LLP, which is serving as counsel to the Fund in connection with the offering, has opined on the legality of the issuance of the common shares offered hereby. Ropes & Gray LLP may rely on the opinion of Richards, Layton & Finger, P.A., Wilmington, Delaware with respect to certain matters of Delaware law.

AVAILABLE INFORMATION

We are subject to the informational requirements of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) and the 1940 Act and are required to file reports, including annual and semi-annual reports, proxy statements and other information with the Commission. These documents are available on the Commission’s EDGAR system and can be inspected and copied for a fee at the Commission’s public reference room, 100 F Street, N.E., Room 1580, Washington, D.C. 20549. Additional information about the operation of the public reference room facilities may be obtained by calling the Commission at (202) 551-5850.

This prospectus supplement and the accompanying prospectus do not contain all of the information in our registration statement, including amendments, exhibits, and schedules. Statements in this prospectus supplement and the accompanying prospectus about the contents of any contract or other document are not necessarily complete and in each instance reference is made to the copy of the contract or other document filed as an exhibit to the registration statement, each such statement being qualified in all respects by this reference.

Additional information about us can be found in our registration statement (including amendments, exhibits, and schedules) on Form N-2 filed with the Commission. The Commission maintains a web site (http://www.sec.gov) that contains our registration statement, other documents incorporated by reference, and other information we have filed electronically with the Commission, including proxy statements and reports filed under the Exchange Act.

Base Prospectus

$75,000,000

Calamos Global Total Return Fund

Common Shares
Preferred Shares
Debt Securities

Calamos Global Total Return Fund (the “Fund,” “we,” “us,” or “our”) is a diversified, closed-end management investment company that commenced investment operations in October 2005. Our investment objective is to provide total return through a combination of capital appreciation and current income.

We may offer, on an immediate, continuous or delayed basis, up to $75,000,000 aggregate initial offering price of our common shares (no par value per share), preferred shares (no par value per share) or debt securities, which we refer to in this prospectus collectively as our securities, in one or more offerings. We may offer our common shares, preferred shares and debt securities separately or together, in amounts, at prices and on terms set forth in a prospectus supplement to this prospectus. You should read this prospectus and the related prospectus supplement carefully before you decide to invest in any of our securities.

We may offer our securities directly to one or more purchasers, through agents that we or they designate from time to time, or to or through underwriters or dealers. The prospectus supplement relating to the particular offering will identify any agents or underwriters involved in the sale of our securities, and will set forth any applicable purchase price, fee, commission or discount arrangement between us and such agents or underwriters or among the underwriters or the basis upon which such amount may be calculated. For more information about the manner in which we may offer our securities, see “Plan of Distribution.” Our securities may not be sold through agents, underwriters or dealers without delivery or deemed delivery of a prospectus supplement and a prospectus.

Our common shares are listed on the NASDAQ Global Select Market under the symbol “CGO.” As of April 30, 2018, the last reported sale price for our common shares was $13.83. As of April 30, 2018, the last reported net asset value for our common shares was $13.05.

Investing in our securities involves certain risks, including the Fund’s use of leverage. You could lose some or all of your investment. See “Risk Factors” beginning on page 41 of this prospectus. Shares of closed-end investment companies frequently trade at a discount to their net asset value and this may increase the risk of loss to purchasers of our securities. You should consider carefully these risks together with all of the other information contained in this prospectus and any prospectus supplement before making a decision to purchase our securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Prospectus dated June 29, 2018

This prospectus, together with any accompanying other prospectus supplement, sets forth concisely the information that you should know before investing. You should read the prospectus and prospectus supplement, which contain important information, before deciding whether to invest in our securities. You should retain the prospectus and prospectus supplement for future reference. A statement of additional information, dated the same date as this prospectus, as supplemented from time to time, containing additional information, has been filed with the Securities and Exchange Commission (“SEC” or the “Commission”) and is incorporated by reference in its entirety into this prospectus. You may request a free copy of the statement of additional information, the table of contents of which is on page 82 of this prospectus, request a free copy of our annual and semi-annual reports, request other information or make shareholder inquiries, by calling toll-free 1-800-582-6959 or by writing to the Fund at 2020 Calamos Court, Naperville, Illinois 60563. The Fund’s annual and semi-annual reports also are available on our website, free of charge, at www.calamos.com, which also provides a link to the Commission’s website, as described below, where the Fund’s statement of additional information can be obtained. Information included on our website does not form part of this prospectus. You can review and copy documents we have filed at the Commission’s Public Reference Room in Washington, D.C. Call 1-202-551-8090 for information. The Commission charges a fee for copies. You can get the same information free from the Commission’s website (http://www.sec.gov). You may also e-mail requests for these documents to publicinfo@sec.gov or make a request in writing to the Commission’s Public Reference Section, Washington, D.C. 20549-0102.

Our securities do not represent a deposit or obligation of, and are not guaranteed or endorsed by, any bank or other insured depository institution and are not federally insured by the Federal Deposit Insurance Corporation, the Federal Reserve Board or any other government agency.

TABLE OF CONTENTS

You should rely only on the information contained or incorporated by reference in this prospectus and any related prospectus supplement in making your investment decisions. We have not authorized any other person to provide you with different or inconsistent information. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus and any prospectus supplement do not constitute an offer to sell or solicitation of an offer to buy any securities in any jurisdiction where the offer or sale is not permitted. The information appearing in this prospectus and in any prospectus supplement is accurate only as of the dates on their covers. Our business, financial condition and prospects may have changed since such dates. We will advise investors of any material changes to the extent required by applicable law.

CAUTIONARY NOTICE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus, any accompanying prospectus supplement and the statement of additional information contain “forward-looking statements.” Forward-looking statements can be identified by the words “may,” “will,” “intend,” “expect,” “estimate,” “continue,” “plan,” “anticipate,” and similar terms and the negative of such terms. Such forward-looking statements may be contained in this prospectus as well as in any accompanying prospectus supplement. By their nature, all forward-looking statements involve risks and uncertainties, and actual results could differ materially from those contemplated by the forward-looking statements. Several factors that could materially affect our actual results are the performance of the portfolio of securities we hold, the price at which our shares will trade in the public markets and other factors discussed in our periodic filings with the Commission. Currently known risk factors that could cause actual results to differ materially from our expectations include, but are not limited to, the factors described in the “Risk Factors” section of this prospectus. We urge you to review carefully that section for a more detailed discussion of the risks of an investment in our securities.

Although we believe that the expectations expressed in our forward-looking statements are reasonable, actual results could differ materially from those projected or assumed in our forward-looking statements. Our future financial condition and results of operations, as well as any forward-looking statements, are subject to change and are subject to inherent risks and uncertainties, such as those disclosed in the “Risk Factors” section of this prospectus. All forward-looking statements contained or incorporated by reference in this prospectus or any accompanying prospectus supplement are made as of the date of this prospectus or the accompanying prospectus supplement, as the case may be. Except for our ongoing obligations under the federal securities laws, we do not intend, and we undertake no obligation, to update any forward-looking statement. The forward-looking statements contained in this prospectus, any accompanying prospectus supplement and the statement of additional information are excluded from the safe harbor protection provided by section 27A of the Securities Act of 1933, as amended (the “1933 Act”).

PROSPECTUS SUMMARY

The following summary contains basic information about us and our securities. It is not complete and may not contain all of the information you may want to consider. You should review the more detailed information contained in this prospectus and in any related prospectus supplement and in the statement of additional information, especially the information set forth under the heading “Risk Factors” beginning on page 41 of this prospectus.

The Fund

The Fund is a diversified, closed-end management investment company. We commenced operations in October 2005 following our initial public offering. As of April 30, 2018, we had $166 million of total managed assets, including $12 million of outstanding mandatory redeemable preferred shares (“MRP Shares”) and $25 million of outstanding borrowings under a liquidity agreement, plus additional structural leverage that amounted to approximately $18 million. Structural leverage refers to borrowings under the liquidity agreement in respect of which the Fund’s interest payments are reduced or eliminated by the Fund’s securities lending activities. See “Leverage.” Our fiscal year ends on October 31. Our investment objective is to provide total return through a combination of capital appreciation and current income.

Investment Adviser

Calamos Advisors LLC (the “Adviser” or “Calamos”) serves as our investment adviser. Calamos is responsible on a day-to-day basis for investment of the Fund’s portfolio in accordance with its investment objective and policies. Calamos makes all investment decisions for the Fund and places purchase and sale orders for the Fund’s portfolio securities. As of April 30, 2018, Calamos managed approximately $21.8 billion in assets of individuals and institutions. Calamos is a wholly-owned subsidiary of Calamos Investments LLC (“CILLC”) and an indirect subsidiary of Calamos Asset Management, Inc.

The Fund pays Calamos an annual fee, payable monthly, for its investment management services equal to 1.00% of the Fund’s average weekly managed assets. “Managed assets” means the total assets of the Fund (including any assets attributable to any leverage that may be outstanding) minus the sum of liabilities (other than debt representing financial leverage). “Net assets” does not include any assets attributable to any leverage that may be outstanding, or other debt representing financial leverage. See “Management of the Fund.”

The principal business address of the Adviser is 2020 Calamos Court, Naperville, Illinois 60563.

The Offering

We may offer, from time to time, in one or more offerings or series, together or separately, up to $75,000,000 of our common shares, preferred shares or debt securities, which we refer to, collectively, as the “securities.” We may sell our securities through underwriters or dealers, “at the market” to or through a market maker into an existing trading market or otherwise directly to one or more purchasers or through agents or through a combination of methods of sale. The identities of such underwriters, dealers, market makers or agents, as the case may be, will be described in one or more supplements to this prospectus. The securities may be offered at prices and on terms to be described in one or more supplements to this prospectus. In the event we offer common shares, the offering price per share of our common shares exclusive of any underwriting commissions or discounts will not be less than the net asset value per share of our common shares at the time we make the offering except as permitted by applicable law. To the extent that the Fund issues common shares and current shareholders do not participate, those current shareholders may experience a dilution of their voting rights as new shares are issued to the public. Depending on the facts, any issuance of new common shares may also have the effect of reducing any premium to per share net asset value at which the shares might trade and the market price at which the shares might trade.

Currently, the Fund does not intend to offer any preferred shares or debt securities (collectively, “senior securities”), but reserves the right to do so in the future.

We may offer our securities directly to one or more purchasers, through agents that we or they designate from time to time, or to or through underwriters or dealers. The prospectus supplement relating to the relevant offering will identify any agents or underwriters involved in the sale of our securities, and will set forth any applicable purchase price, fee, commission or discount arrangement between us and such agents or underwriters or among underwriters and the basis upon which such amount may be calculated. See “Plan of Distribution.” Our securities may not be sold through agents, underwriters or dealers without delivery or deemed delivery of a prospectus and prospectus supplement describing the method and terms of the applicable offering of our securities.

Use of Proceeds

Unless otherwise specified in a prospectus supplement, we currently intend to use the net proceeds from the sale of our securities primarily to invest in accordance with our investment objective and policies within approximately three months of receipt of such proceeds. We may also use proceeds from the sale of our securities (i) to retire all or a portion of any short-term debt we incur in pursuit of our investment objective and policies, (ii) to redeem any outstanding senior securities, and (iii) for working capital purposes, including the payment of interest and operating expenses, although there is currently no intent to issue securities primarily for these purposes.

Dividends and Distributions on Common Shares

The Fund intends to distribute to common shareholders all or a portion of its net investment income monthly and net realized capital gains, if any, at least annually. On November 4, 2008, the Commission granted Calamos, on behalf of itself and certain closed-end funds that it manages, including the Fund, or may manage in the future, an order granting an exemption from Section 19(b) of, and Rule 19b-1 under, the Investment Company Act of 1940, as amended (the “1940 Act”), to conditionally permit the Fund to make periodic distributions of long-term capital gains with respect to the Fund’s outstanding common shares as frequently as twelve times each year, so long as it complies with the conditions of the order and maintains in effect a distribution policy with respect to its common shares calling for periodic distributions of an amount equal to a fixed amount per share, a fixed percentage of market price per share or a fixed percentage of the Fund’s net asset value per share (a “Managed Distribution Policy”). See “Dividends and Distributions on Common Shares; Automatic Dividend Reinvestment Plan — Dividends and Distributions on Common Shares” for a discussion of the requirements under the order permitting the Managed Distribution Policy.

As of January 1, 2018, the Fund adopted such Managed Distribution Policy. Pursuant to such policy, the Fund currently intends to make monthly distributions to common shareholders stated in terms of a fixed cent per common share distribution rate that would be composed of, in addition to net investment income, supplemental amounts generally representing realized capital gains or, possibly, returns of capital representing either unrealized capital gains or a return of original investment. Such distributions, including such supplemental amounts, are sometimes referred to as “managed distributions.”

The Fund will seek to establish a distribution rate that roughly corresponds to the Adviser’s projections of the total return that could reasonably be expected to be generated by the Fund over an extended period of time, although the distribution rate will not be solely dependent on the amount of income earned or capital gains realized by the Fund. Calamos, in making such projections, may consider long-term historical returns and a variety of other factors. If, for any monthly distribution, net investment income and net realized capital gains were less than the amount of the distribution, the difference would be distributed from the Fund’s assets. In addition, in order to make such distributions, the Fund might have to sell a portion of its investment portfolio at a time when independent investment judgment might not dictate such action. The Fund’s final distribution for each calendar year will include any remaining net investment income undistributed during the year and may include any remaining net realized capital gains undistributed during the year. The Fund’s actual financial performance will likely vary significantly from quarter to quarter and from year to year, and there may be extended periods of up to several years when the distribution rate will exceed the Fund’s actual total returns. The Fund’s projected or actual distribution rate is not a prediction of what the Fund’s actual total returns will

be over any specific future period. See “Certain Federal Income Tax Matters — Federal Income Taxation of Common and Preferred Shareholders” and “Dividends and Distributions on Common Shares; Automatic Dividend Reinvestment Plan — Dividends and Distributions on Common Shares” below for a discussion of the short- and long-term implications associated with Fund distributions.

As portfolio and market conditions change, the rate of distributions on the common shares and the Fund’s distribution policy could change. To the extent that the total return of the Fund exceeds the distribution rate for an extended period, the Fund may be in a position to increase the distribution rate or distribute supplemental amounts to shareholders. Conversely, if the total return of the Fund is less than the distribution rate for an extended period of time, the Fund will effectively be drawing upon its net assets to meet payments prescribed by its distribution policy. The rate may be modified by the Fund’s Board from time to time without prior notice to the Fund’s shareholders.

Net realized short-term capital gains distributed to shareholders will be taxed as ordinary income for federal income tax purposes and net realized long-term capital gain (if any) will be taxed for federal income tax purposes at long-term capital gain rates. To the extent the Fund distributes an amount in excess of the Fund’s current and accumulated earnings and profits, such excess, if any, will be treated by a shareholder for federal income tax purposes as a tax-free return of capital to the extent of the shareholder’s adjusted tax basis in his, her or its shares and thereafter as a gain from the sale or exchange of such shares. Any such distributions made by the Fund will reduce the shareholder’s adjusted tax basis in his, her or its shares to the extent that the distribution constitutes a return of capital on a tax basis during any calendar year and, thus, could potentially subject the shareholder to capital gains taxation in connection with a later sale of Fund shares, even if those shares are sold at a price that is lower than the shareholder’s original investment price. To the extent that the Fund’s distributions exceed the Fund’s current and accumulated earnings and profits, the distribution payout rate will exceed the yield generated from the Fund’s investments. There is no guarantee that the Fund will realize capital gain in any given year. Distributions are subject to re-characterization for federal income tax purposes after the end of the fiscal year. See “Certain Federal Income Tax Matters.”

Pursuant to the Fund’s Automatic Dividend Reinvestment Plan, unless a shareholder is ineligible or elects otherwise, all dividends and capital gain distributions on common shares are automatically reinvested in additional common shares of the Fund. However, an investor can choose to receive dividends and distributions in cash. Since investors can participate in the automatic dividend reinvestment plan only if their broker or nominee participates in our plan, you should contact your broker or nominee to confirm that you are eligible to participate in the plan. See “Dividends and Distributions on Common Shares; Automatic Dividend Reinvestment Plan — Automatic Dividend Reinvestment Plan.”

Investment Policies

Primary Investments. Under normal circumstances, the Fund will invest primarily in a portfolio of common and preferred stocks, convertible securities and income producing securities such as investment grade and below investment grade (high yield/high risk) debt securities. The Fund, under normal circumstances, will invest at least 50% of its managed assets in equity securities (including securities that are convertible into equity securities). The Fund may invest up to 100% of its managed assets in securities of foreign issuers, including debt and equity securities of corporate issuers and debt securities of government issuers, in developed and emerging markets. Under normal circumstances, the Fund will invest at least 40% of its managed assets in securities of foreign issuers. The Fund will invest in the securities of issuers of several different countries throughout the world, in addition to the United States.

Calamos will dynamically allocate the Fund’s investments among multiple asset classes (rather than maintaining a fixed or static allocation), seeking to obtain an appropriate balance of risk and reward through all market cycles using multiple strategies and combining them to seek to achieve favorable risk adjusted returns.

The Fund will attempt to keep a consistent balance between risk and reward over the course of different market cycles, through various combinations of stocks, bonds, and/or convertible securities, to achieve what Calamos believes to be an appropriate blend for the then current market. As the market environment changes,

portfolio securities may change in an attempt to achieve a relatively consistent risk level over time. At some points in a market cycle, one type of security may make up a substantial portion of the Fund’s portfolio, while at other times certain securities may have minimal or no representation, depending on market conditions.

The Fund may also seek to generate income from option premiums by writing (selling) options (with an aggregate notional value of up to 33% of the value of the Fund’s managed assets). The Fund will opportunistically employ a strategy of writing options. The extent of option writing activity will depend upon market conditions and Calamos’ ongoing assessment of the attractiveness of writing options on the Fund’s equity holdings.

The Fund’s derivative activities are principally focused on the following derivatives: interest rate swaps, convertible securities, synthetic convertible securities, options on individual securities, index options and forward currency exchange contracts (“forward contracts”). However, the Fund reserves the right to invest in other derivative instruments to the extent it is consistent with the Fund’s investment objective and restrictions.

Equity Securities. Equity securities include common and preferred stocks, warrants, rights, and depository receipts. Under normal circumstances, the Fund will invest at least 50% of its managed assets in equity securities (including securities that are convertible into equity securities). See “— Convertible Securities” below. The Fund may invest in preferred stocks and convertible securities of any rating, including below investment grade. See “— High Yield Securities” below. Equity securities, such as common stock, generally represent an ownership interest in a company. Therefore, the Fund participates in the financial success or failure of any company in which it has an equity interest. Although equity securities have historically generated higher average returns than fixed income securities, equity securities have also experienced significantly more volatility in those returns. An adverse event, such as an unfavorable earnings report, may depress the value of a particular equity security held by the Fund. Also, the price of equity securities, particularly common stocks, are sensitive to general movements in the stock market. A drop in the stock market may depress the price of equity securities held by the Fund.

Debt Securities. The Fund may invest in debt securities, including debt securities of U.S. and foreign corporate issuers (also known as corporate bonds). Holders of corporate bonds, as creditors, have a prior legal claim over common and preferred stockholders as to both income and assets of the issuer for the principal and interest due them and may have a prior claim over other creditors if liens or mortgages are involved. Interest on corporate bonds may be fixed or floating, or the securities may be zero coupon fixed income securities which pay no interest. Corporate bonds contain elements of both interest rate risk and credit risk. The market value of a corporate bond generally may be expected to rise and fall inversely with changes in interest rates and may also be affected by the credit rating of the issuer, the issuer’s performance and perceptions of the issuer in the marketplace.

High Yield Securities. The Fund may invest in high yield securities for either current income or capital appreciation or both. These securities are rated below investment grade—i.e., rated “Ba” or lower by Moody’s Investors Service, Inc. (“Moody’s”) or “BB” or lower by Standard & Poor’s Financial Services, LLC, a subsidiary of The McGraw-Hill Companies, Inc. (“Standard & Poor’s”), or are unrated securities of comparable quality as determined by Calamos, the Fund’s investment adviser. The Fund may invest in high yield securities of any rating. Non-convertible debt securities rated below investment grade are commonly referred to as “junk bonds” and are considered speculative with respect to the issuer’s capacity to pay interest and repay principal. They involve greater risk of loss, are subject to greater price volatility and are less liquid, especially during periods of economic uncertainty or change, than higher rated securities.

Foreign Securities. The Fund may invest up to 100% of its managed assets in securities of foreign issuers in developed and emerging markets, including debt and equity securities of corporate issuers and debt securities of government issuers. Under normal circumstances, the Fund will invest at least 40% of its managed assets in securities of foreign issuers. The Fund will invest in the securities of issuers of several different countries throughout the world, in addition to the United States. A foreign issuer is a foreign government or a company organized under the laws of a foreign country. In analyzing the foreign issuers in which the Fund may invest, Calamos will generally consider a number of factors that may characterize the issuer’s economic ties to a

particular foreign country or region. Such factors may include any or all of the following: the characteristics of the economy in the principal country or countries in which the issuer sells it goods and/or services; the stability of the currency in the issuer’s country of organization; the laws with respect to international trade and property rights in the issuer’s country of organization; and the tax, accounting and regulatory requirements of the issuer’s country of organization. See “Investment Objective and Principal Investment Strategies — Principal Investment Strategies — Foreign Securities.”

Convertible Securities. The Fund may invest in convertible securities. A convertible security is a debt security or preferred stock that is exchangeable for an equity security (typically of the same issuer) at a predetermined price (the “conversion price”). Depending upon the relationship of the conversion price to the market value of the underlying security, a convertible security may trade more like an equity security than a debt instrument. The Fund may invest in convertible securities of any rating. Securities that are convertible into equity securities are considered equity securities for purposes of the Fund’s policy to invest at least 50% of its managed assets in equity securities.

Synthetic Convertible Securities. The Fund may invest in “synthetic” convertible securities. A synthetic convertible security is a financial instrument that is designed to simulate the characteristics of another instrument (i.e., a convertible security) through the combined features of a collection of other securities or assets. Calamos may create a synthetic convertible security by combining separate securities that possess the two principal characteristics of a true convertible security, i.e., a fixed-income security (“fixed-income component”, which may be a convertible or non-convertible security) and the right to acquire an equity security (“convertible component”). The fixed-income component is achieved by investing in fixed-income securities such as bonds, preferred stocks and money market instruments. The convertible component is achieved by investing in warrants or options to buy common stock at a certain exercise price, or options on a stock index.

The Fund may also invest in synthetic convertible securities created by third parties, typically investment banks. Synthetic convertible securities created by such parties may be designed to simulate the characteristics of traditional convertible securities or may be designed to alter or emphasize a particular feature. Traditional convertible securities typically offer the opportunity for stable cash flows with the ability to participate in capital appreciation of the underlying common stock. Traditional convertible securities are exercisable at the option of the holder. Synthetic convertible securities may alter these characteristics by offering enhanced yields in exchange for reduced capital appreciation or additional risk of loss, or any combination of these features. Synthetic convertible instruments may include structured notes, equity-linked notes, mandatory convertibles and combinations of securities and instruments, such as a debt instrument combined with a forward contract. The Fund’s holdings of synthetic convertible securities are considered equity securities for purposes of the Fund’s policy to invest at least 50% of its managed assets in equity securities. If the Fund purchases a synthetic convertible security, a component of which is an option, such option will not be considered an option for the purpose of the Fund’s limitations on options described below. See “Investment Objective and Principal Investment Strategies — Principal Investment Strategies — Synthetic Convertible Securities.”

Options Writing. The Fund may seek to generate income from option premiums by writing (selling) options (with an aggregate notional value of up to 33% of the value of the Fund’s managed assets). The Fund may write (sell) call options (i) on a portion of the equity securities (including securities that are convertible into equity securities) in the Fund’s portfolio and (ii) on broad-based securities indices (such as the Standard and Poor’s 500^® Index (“S&P 500”) or the MSCI EAFE^® Index (“MSCI EAFE”), which is an index of international equity stocks) or certain ETFs (exchange-traded funds) that trade like common stocks but seek to replicate such market indices. See “Investment Objective and Principal Investment Strategies — Principal Investment Strategies — Options Writing” and “Investment Objective and Principal Investment Strategies — Principal Investment Strategies — Options in General.”

Rule 144A Securities. The Fund may invest without limit in certain securities (“Rule 144A Securities”), such as convertible and debt securities, that are typically purchased in transactions exempt from the registration requirements of the 1933 Act pursuant to Rule 144A under that Act. Rule 144A Securities may only be sold to qualified institutional buyers, such as the Fund. Any resale of these securities must generally be effected through a sale that is registered under the 1933 Act or otherwise exempted or excepted from such registration requirements. Under the supervision and oversight of the Fund’s Board of Trustees, Calamos will determine whether Rule 144A Securities are liquid. Typically, the Fund purchases Rule 144A Securities only if Calamos has determined them to be liquid. If any Rule 144A Security held by the Fund should become illiquid, the value of the security may be reduced and a sale of the security may be more difficult. See “Investment Objective and Principal Investment Strategies — Principal Investment Strategies — Rule 144A Securities.”

Zero Coupon Securities. The securities in which the Fund invests may include zero coupon securities, which are debt obligations that are issued or purchased at a significant discount from face value. The discount approximates the total amount of interest the security will accrue and compound over the period until maturity or the particular interest payment date at a rate of interest reflecting the market rate of the security at the time of issuance. Zero coupon securities do not require the periodic payment of interest. These investments benefit the issuer by mitigating its need for cash to meet debt service, but generally require a higher rate of return to attract investors who are willing to defer receipt of cash. These investments may experience greater volatility in market value than U.S. government or other securities that make regular payments of interest. The Fund accrues income on these investments for tax and accounting purposes, which is distributable to shareholders and which, because no cash is received at the time of accrual, may require the liquidation of other portfolio securities to satisfy the Fund’s distribution obligations, in which case the Fund will forego the opportunity to purchase additional income producing assets with the liquidation proceeds. Zero coupon U.S. government securities include STRIPS and CUBES, which are issued by the U.S. Treasury as component parts of U.S. Treasury bonds and represent scheduled interest and principal payments on the bonds. See “Investment Objective and Principal Investment Strategies — Principal Investment Strategies — Zero Coupon Securities.”

Other Securities. The Fund may invest in other securities of various types to the extent consistent with its investment objective. Normally, the Fund invests substantially all of its assets to meet its investment objective. For temporary defensive purposes, the Fund may depart from its principal investment strategies and invest part or all of its assets in securities with remaining maturities of less than one year or cash equivalents; or it may hold cash. During such periods, the Fund may not be able to achieve its investment objective. There are no restrictions as to the ratings of debt securities acquired by the Fund or the portion of the Fund’s assets that may be invested in debt securities in a particular ratings category. For more information on the types of derivatives that the Fund invests in, see “Investment Objective and Principal Investment Strategies — Principal Investment Strategies” in this prospectus and “Investment Objective and Policies” in the statement of additional information.

See “Investment Objective and Principal Investment Strategies — Principal Investment Strategies” for a further discussion of the primary investments and principal investment strategies of the Fund.

Use of Leverage by the Fund

The Fund currently uses, and may in the future use, financial leverage. The Fund has obtained financial leverage (i) under an Amended and Restated Liquidity Agreement (the “SSB Agreement”) with State Street Bank and Trust Company (“SSB”) that allows the Fund to borrow up to $55 million and (ii) through the issuance of three series of mandatory redeemable preferred shares (“MRP Shares”) with an aggregate liquidation preference of $12.0 million, as described in greater detail below. The SSB Agreement provides for securities lending and securities repurchase transactions that may offset some of the interest rate payments that would otherwise be due in respect of the borrowings under the SSB Agreement. The Fund’s outstanding MRP Shares include 160,000 Series A MRP Shares, with an aggregate liquidation preference of $4.0 million and a mandatory redemption date of September 6, 2022; 160,000 Series B MRP Shares, with an aggregate liquidation preference of $4.0 million and a mandatory redemption date of September 6, 2024; and 160,000 Series C MRP Shares, with an aggregate liquidation preference of $4.0 million and a mandatory redemption date of September 6, 2027.

The Series A, Series B and Series C MRP Shares are to pay monthly cash dividends initially at rates of 3.70%, 4.00% and 4.24%, respectively, subject to adjustment under certain circumstances. Additional details regarding the SSB Agreement and the MRP Shares are included under “Leverage.”

As of April 30, 2018, the Fund had utilized $43 million of the $55 million available under the SSB Agreement ($25 million in borrowings outstanding, and $18 million in structural leverage consisting of collateral received from SSB in connection with securities on loan), representing 25.9% of managed assets as of that date, and had $12 million of MRP Shares outstanding, representing 7.2% of managed assets. Combined, the borrowings under the SSB Agreement and the outstanding MRP Shares represented 33.2% of managed assets.

The Fund may make further use of financial leverage through the issuance of additional preferred shares or may borrow money or issue additional debt securities to the extent permitted under the 1940 Act or under the SSB Agreement. As a non-fundamental policy, the Fund may not issue preferred shares or borrow money and/or issue debt securities with an aggregate liquidation preference and aggregate principal amount exceeding 38% of the Fund’s total assets measured at the time of borrowing or issuance of the new securities. However, the Board of Trustees reserves the right to issue preferred shares or debt securities or borrow to the extent permitted by the 1940 Act. See “Leverage.” The holders of preferred shares or debt, if any, on the one hand, and the holders of the common shares, on the other, may have interests that conflict with each other in certain situations. See “Description of Securities — Preferred Shares” and “Certain Provisions of the Agreement and Declaration of Trust and By-Laws, Including Antitakeover Provisions.”

Because Calamos’ investment management fee is a percentage of the Fund’s managed assets, Calamos’ fee will be higher if the Fund is leveraged and Calamos will have an incentive to be more aggressive and leverage the Fund. Consequently, the Fund and Calamos may have differing interests in determining whether to leverage the Fund’s assets. Any additional use of leverage by the Fund effected through new, additional or increased credit facilities or the issuance of preferred shares would require approval by the Board of Trustees of the Fund. In considering whether to approve the use of additional leverage through those means, the Board would be presented with all relevant information necessary to make a determination whether or not additional leverage would be in the best interests of the Fund, including information regarding any potential conflicts of interest. For further information about the Fund’s financial leverage, see “Use of Leverage by the Fund.”

For further information about the effects of the Fund’s financial leverage and an illustration of the hypothetical effect on the return to a holder of the Fund’s common shares of the leverage obtained by borrowing under the Fund’s financing package, see “Effects of Leverage.” For further information about leveraging, see “Risk Factors — Fund Risks — Leverage Risk.”

Interest Rate Transactions

In order to seek to reduce the interest rate risk inherent in the Fund’s underlying investments and capital structure, the Fund, if Calamos deems market conditions favorable, may enter into over-the-counter interest rate swap or cap transactions to attempt to protect itself from increasing dividend or interest expenses on its leverage. The use of interest rate swaps and caps is a highly specialized activity that involves investment techniques and risks different from those associated with ordinary portfolio security transactions.

In an interest rate swap, the Fund would agree to pay to the other party to the interest rate swap (which is known as the “counterparty”) a fixed rate payment in exchange for the counterparty agreeing to pay to the Fund a payment at a variable rate that is expected to approximate the rate on any variable rate payment obligation on the Fund’s leverage. The payment obligations would be based on the notional amount of the swap.

In an interest rate cap, the Fund would pay a premium to the counterparty to the interest rate cap and, to the extent that a specified variable rate index exceeds a predetermined fixed rate, would receive from the counterparty payments of the difference based on the notional amount of such cap. There can be no assurance that the Fund will use interest rate transactions or that, if used, their use will be beneficial to the Fund. Depending on the state of interest rates in general, the Fund’s use of interest rate swap or cap transactions could enhance or harm the overall performance of the common shares. See “Interest Rate Transactions.”

Forward Currency Exchange Transactions

The Fund may use forward currency exchange contracts. Forward contracts are contractual agreements to purchase or sell a specified currency at a specified future date (or within a specified time period) and price set at the time of the contract. Forward contracts are usually entered into with banks, foreign exchange dealers and broker-dealers, are not exchange traded, and are usually for less than one year, but may be renewed. 

Forward currency exchange transactions may involve currencies of the different countries in which the Fund may invest and serve as hedges against possible variations in the exchange rate between these currencies and the U.S. dollar. Currency exchange transactions are limited to transaction hedging and portfolio hedging involving either specific transactions or portfolio positions, except to the extent described in the statement of additional information under “Investment Objective and Policies — Synthetic Foreign Money Market Positions.” Transaction hedging is the purchase or sale of forward contracts with respect to specific receivables or payables of the Fund accruing in connection with the purchase and sale of its portfolio securities or the receipt of dividends or interest thereon. Portfolio hedging is the use of forward contracts with respect to portfolio security positions denominated or quoted in a particular foreign currency. Portfolio hedging allows the Fund to limit or reduce its exposure in a foreign currency by entering into a forward contract to sell such foreign currency (or another foreign currency that acts as a proxy for that currency) at a future date for a price payable in U.S. dollars so that the value of the foreign denominated portfolio securities can be approximately matched by a foreign denominated liability. The Fund may not engage in portfolio hedging with respect to the currency of a particular country to an extent greater than the aggregate market value (at the time of making such sale) of the securities held in its portfolio denominated or quoted in that particular currency, except that the Fund may hedge all or part of its foreign currency exposure through the use of a basket of currencies or a proxy currency where such currencies or currency act as an effective proxy for other currencies. In such a case, the Fund may enter into a forward contract where the amount of the foreign currency to be sold exceeds the value of the securities denominated in such currency. The use of this basket hedging technique may be more efficient and economical than entering into separate forward contracts for each currency held in the Fund. The Fund may not engage in “speculative” currency exchange transactions.

Hedging against a decline in the value of a currency does not eliminate fluctuations in the value of a portfolio security traded in that currency or prevent a loss if the value of the security declines. Hedging transactions also preclude the opportunity for gain if the value of the hedged currency should rise. Moreover, it may not be possible for the Fund to hedge against a devaluation that is so generally anticipated that the Fund is not able to contract to sell the currency at a price above the devaluation level it anticipates. The cost to the Fund of engaging in currency exchange transactions varies with such factors as the currency involved, the length of the contract period, and prevailing market conditions. See “Investment Objective and Principal Investment Strategies — “Forward Currency Exchange Transactions.”

Conflicts of Interest

Conflicts of interest may arise from the fact that Calamos and its affiliates carry on substantial investment activities for other clients, in which the Fund does not have an interest. Calamos or its affiliates may have financial incentives to favor certain of these accounts over the Fund. Any of their proprietary accounts or other customer accounts may compete with the Fund for specific trades. Calamos or its affiliates may give advice and recommend securities to, or buy or sell securities for, other accounts and customers, which advice or securities recommended may differ from advice given to, or securities recommended or bought or sold for, the Fund, even though their investment objectives may be the same as, or similar to, the Fund’s objective.

Situations may occur when the Fund could be disadvantaged because of the investment activities conducted by Calamos and its affiliates for their other accounts. Such situations may be based on, among other things, the following: (1) legal or internal restrictions on the combined size of positions that may be taken for the Fund or the other accounts, thereby limiting the size of the Fund’s position; or (2) the difficulty of liquidating an investment for the Fund or the other accounts where the market cannot absorb the sale of the combined position. See “Investment Objective and Principal Investment Strategies — Conflicts of Interest.”

Fund Risks

Equity Securities Risk. Equity investments are subject to greater fluctuations in market value than other asset classes as a result of such factors as the issuer’s business performance, investor perceptions, stock market trends and general economic conditions. Equity securities are subordinated to bonds and other debt instruments in a company’s capital structure in terms of priority to corporate income and liquidation payments. The Fund may invest in preferred stocks and convertible securities of any rating, including below investment grade. See “Risk Factors — Fund Risks — Equity Securities Risk.”

Debt Securities Risk. The Fund may invest in debt securities, including corporate bonds and high yield securities. In addition to the risks described elsewhere in this prospectus (such as high yield securities risk and interest rate risk), debt securities are subject to certain additional risks, including issuer risk and reinvestment risk. Issuer risk is the risk that the value of debt securities may decline for a number of reasons which directly relate to the issuer, such as management performance, leverage and reduced demand for the issuer’s goods and services. Reinvestment risk is the risk that income from the Fund’s portfolio will decline if the Fund invests the proceeds from matured, traded or called bonds at market interest rates that are below the Fund portfolio’s current earnings rate. A decline in income could affect the market price of the Fund’s common shares or the overall return of the Fund.

High Yield Securities Risk. The Fund may invest in high yield securities of any rating. Investment in high yield securities involves substantial risk of loss. Below investment grade non-convertible debt securities or comparable unrated securities are commonly referred to as “junk bonds” and are considered predominantly speculative with respect to the issuer’s ability to pay interest and principal and are susceptible to default or decline in market value due to adverse economic and business developments. The market values for high yield securities tend to be very volatile, and these securities are less liquid than investment grade debt securities. For these reasons, your investment in the Fund is subject to the following specific risks:

•increased price sensitivity to changing interest rates and to a deteriorating economic environment;

•greater risk of loss due to default or declining credit quality;

•adverse company specific events are more likely to render the issuer unable to make interest and/or principal payments; and

•if a negative perception of the high yield market develops, the price and liquidity of high yield securities may be depressed. This negative perception could last for a significant period of time.

Adverse changes in economic conditions are more likely to lead to a weakened capacity of a high yield issuer to make principal payments and interest payments than an investment grade issuer. The principal amount of high yield securities outstanding has proliferated since the inception of the Fund as an increasing number of issuers have used high yield securities for corporate financing. An economic downturn could severely affect the ability of highly leveraged issuers to service their debt obligations or to repay their obligations upon maturity.

The secondary market for high yield securities may not be as liquid as the secondary market for more highly rated securities, a factor which may have an adverse effect on the Fund’s ability to dispose of a particular security. There are fewer dealers in the market for high yield securities than for investment grade obligations. The prices quoted by different dealers may vary significantly and the spread between the bid and asked price is generally much larger than for higher quality instruments. Under adverse market or economic conditions, the secondary market for high yield securities could contract further, independent of any specific adverse changes in the condition of a particular issuer, and these instruments may become illiquid. As a result, the Fund could find it more difficult to sell these securities or may be able to sell the securities only at prices lower than if such securities were widely traded. Prices realized upon the sale of such lower rated or unrated securities, under these circumstances, may be less than the prices used in calculating the Fund’s net asset value. See “Risk Factors — Fund Risks — High Yield Securities Risk.”

Foreign Securities Risk. Investments in non-U.S. issuers may involve unique risks compared to investing in securities of U.S. issuers. These risks are more pronounced to the extent that the Fund invests a significant portion of its non-U.S. investments in one region or in the securities of emerging market issuers. See also “— Emerging Markets Risk” below. These risks may include:

•less information may be available about non-U.S. issuers or markets due to less rigorous disclosure or accounting standards or regulatory practices in foreign jurisdictions;

•many non-U.S. markets are smaller, less liquid and more volatile. In a changing market, Calamos may not be able to sell the Fund’s portfolio securities at times, in amounts and at prices it considers reasonable;

•an adverse effect of currency exchange rate changes or controls on the value of the Fund’s investments;

•the economies of non-U.S. countries may grow at slower rates than expected or may experience a downturn or recession;

•economic, political and social developments may adversely affect the securities markets in foreign jurisdictions, including expropriation and nationalization;

•the difficulty in obtaining or enforcing a court judgment in non-U.S. countries;

•restrictions on foreign investments in non-U.S. jurisdictions;

•difficulties in effecting the repatriation of capital invested in non-U.S. countries;

•withholding and other non-U.S. taxes may decrease the Fund’s return; and

•dividend income the Fund receives from foreign securities may not be eligible for the special tax treatment applicable to qualified dividend income.

Based upon the Fund’s test for determining whether an issuer is a “foreign issuer” as described above, it is possible that an issuer of securities in which the Fund invests could be organized under the laws of a foreign country, yet still conduct a substantial portion of its business in the U.S. or have substantial assets in the U.S.  In this case, such a “foreign issuer” may be subject to the market conditions in the U.S. to a greater extent than it may be subject to the market conditions in the country of its organization. See “Risk Factors — Fund Risks — Foreign Securities Risk.” See also “— Non-U.S. Government Obligation Risk.”

Emerging Markets Risk. Emerging market countries may have relatively unstable governments and economies based on only a few industries, which may cause greater instability. The value of emerging market securities will likely be particularly sensitive to changes in the economics of such countries. These countries are also more likely to experience higher levels of inflation, deflation or currency devaluations, which could adversely affect the value of the Fund’s investments and hurt those countries’ economies and securities markets. See “Risk Factors — Fund Risks — Emerging Markets Risk.”

Currency Risk. The value of the securities denominated or quoted in foreign currencies may be adversely affected by fluctuations in the relative currency exchange rates and by exchange control regulations. The Fund’s investment performance may be negatively affected by a devaluation of a currency in which the Fund’s investments are denominated or quoted. Further, the Fund’s investment performance may be significantly affected, either positively or negatively, by currency exchange rates because the U.S. dollar value of securities denominated or quoted in another currency will increase or decrease in response to changes in the value of such currency in relation to the U.S. dollar. See “Risk Factors — Fund Risks — Currency Risk.”

Interest Rate Risk. In addition to the risks discussed above, debt securities, including high yield securities, are subject to certain risks, including:

•if interest rates go up, the value of debt securities in the Fund’s portfolio generally will decline;

•during periods of declining interest rates, the issuer of a security may exercise its option to prepay principal earlier than scheduled, forcing the Fund to reinvest in lower yielding securities. This is known as call or prepayment risk. Debt securities frequently have call features that allow the issuer to repurchase the security prior to its stated maturity. An issuer may redeem an obligation if the issuer can refinance the debt at a lower cost due to declining interest rates or an improvement in the credit standing of the issuer;

•during periods of rising interest rates, the average life of certain types of securities may be extended because of slower than expected principal payments. This may lock in a below market interest rate, increase the estimated period until the security is paid in full and reduce the value of the security. This is known as extension risk;

•rising interest rates could result in an increase in the cost of the Fund’s leverage and could adversely affect the ability of the Fund to meet asset coverage requirements with respect to leverage; and

•the risks associated with rising interest rates may be particularly acute in the current market environment because market interest rates currently are near historically low levels. However, continued economic recovery, the end of the Federal Reserve Board’s quantitative easing program, and an increased likelihood of a rising interest rate environment increase the risk that interest rates will continue to rise in the near future.

See “Risk Factors — Fund Risks — Interest Rate Risk.”

Non-U.S. Government Obligation Risk. An investment in debt obligations of non-U.S. governments and their political subdivisions involves special risks that are not present in corporate debt obligations. The non-U.S. issuer of the sovereign debt or the non-U.S. governmental authorities that control the repayment of the debt may be unable or unwilling to repay principal or interest when due, and the Fund may have limited recourse in the event of a default. During periods of economic uncertainty, the market prices of sovereign debt may be more volatile than prices of debt obligations of U.S. issuers.

Leverage Risk. The Fund has issued indebtedness and preferred shares and may borrow money or issue debt securities. As of April 30, 2018, the Fund has leverage in the form of borrowings under the SSB Agreement and outstanding MRP Shares. Leverage is the potential for the Fund to participate in gains and losses on an amount that exceeds the Fund’s investment. The borrowing of money or issuance of debt securities and preferred shares represents the leveraging of the Fund’s common shares. As a non-fundamental policy, the Fund may not issue preferred shares or borrow money and/or issue debt securities with an aggregate liquidation preference and aggregate principal amount exceeding 38% of the Fund’s total assets as measured at the time of borrowing or issuance of the new securities. However, the Board of Trustees reserves the right to issue preferred shares or debt securities or borrow to the extent permitted by the 1940 Act. See “Leverage.”

Leverage creates risks which may adversely affect the return for the holders of common shares, including:

•the likelihood of greater volatility in the net asset value and market price of the Fund’s common shares;

•fluctuations in the dividend rates on any preferred shares borne by the Fund or in interest rates on borrowings and short-term debt;

•increased operating costs, which are effectively borne by common shareholders, may reduce the Fund’s total return; and

•the potential for a decline in the value of an investment acquired with borrowed funds, while the Fund’s obligations under such borrowing or preferred shares remain fixed.

In addition, the rights of lenders and the holders of preferred shares and debt securities issued by the Fund will be senior to the rights of the holders of common shares with respect to the payment of dividends or to the payment of assets upon liquidation. Holders of preferred shares have voting rights in addition to and separate from the voting rights of common shareholders. See “Description of Securities — Preferred Shares”

and “Certain Provisions of the Agreement and Declaration of Trust and By-Laws, Including Antitakeover Provisions.” The holders of preferred shares or debt, if any, on the one hand, and the holders of the common shares, on the other, may have interests that conflict in certain situations.

Leverage is a speculative technique that could adversely affect the returns to common shareholders. Leverage can cause the Fund to lose money and can magnify the effect of any losses. To the extent the income or capital appreciation derived from securities purchased with funds received from leverage exceeds the cost of leverage, the Fund’s return will be greater than if leverage had not been used. Conversely, if the income or capital appreciation from the securities purchased with such funds is not sufficient to cover the cost of leverage or if the Fund incurs capital losses, the return of the Fund will be less than if leverage had not been used, and therefore the amount available for distribution to common shareholders as dividends and other distributions will be reduced or potentially eliminated.

The Fund will pay, and common shareholders will effectively bear, any costs and expenses relating to any borrowings and to the issuance and ongoing maintenance of preferred shares or debt securities. Such costs and expenses include the higher management fee resulting from the use of any such leverage, offering and/or issuance costs, and interest and/or dividend expense and ongoing maintenance. These conditions may, directly or indirectly, result in higher leverage costs to common shareholders.

Certain types of borrowings may result in the Fund being subject to covenants in credit agreements, including those relating to asset coverage, borrowing base and portfolio composition requirements and additional covenants that may affect the Fund’s ability to pay dividends and distributions on common shares in certain instances. The Fund may also be required to pledge its assets to the lenders in connection with certain types of borrowings. The Fund may be subject to certain restrictions on investments imposed by guidelines of rating agencies which may issue ratings for the preferred shares or short-term debt instruments issued by the Fund. These guidelines may impose asset coverage or portfolio composition requirements that are more stringent than those imposed by the 1940 Act. The Board reserves the right to change the amount and type of leverage that the Fund uses, and reserves the right to implement changes to the Fund’s borrowings that it believes are in the best interests of the Fund, even if such changes impose a higher interest rate or other costs or impacts over the intermediate, or short-term time period. There is no guarantee that the Fund will maintain leverage at the current rate, and the Board reserves the right to raise, decrease, or eliminate the Fund’s leverage exposure. See “Prospectus Summary — Use of Leverage by the Fund.”

Default Risk. Default risk refers to the risk that a company that issues a convertible or debt security will be unable to fulfill its obligations to repay principal and interest. The lower a debt security is rated, the greater its default risk. The Fund may incur cost and delays in enforcing its rights against the defaulting issuer. See “Risk Factors — Fund Risks — Default Risk.”

Liquidity Risk. The Fund may invest up to 15% of its managed assets in securities that, at the time of investment, are illiquid (i.e., securities that cannot be disposed of within 7 days in the ordinary course of business at approximately the value at which the Fund has valued the securities). The Fund may also invest without limit in Rule 144A Securities determined to be liquid. Calamos, under the supervision and oversight of the Board of Trustees, will determine whether Rule 144A Securities are illiquid (that is, not readily marketable) and thus subject to the Fund’s limit on investing no more than 15% of its managed assets in illiquid securities. Investments in Rule 144A Securities could have the effect of increasing the amount of the Fund’s assets invested in illiquid securities if qualified institutional buyers are unwilling to purchase these Rule 144A Securities. Illiquid securities may be difficult to dispose of at a fair price at the times when the Fund believes it is desirable to do so. Investment of the Fund’s assets in illiquid securities may restrict the Fund’s ability to take advantage of market opportunities. The market price of illiquid securities generally is more volatile than that of more liquid securities, which may adversely affect the price that the Fund pays for or recovers upon the sale of illiquid securities. Illiquid securities are also more difficult to value and may be fair valued by the Board, in which case Calamos’ judgment may play a greater role in the valuation process. The risks associated with illiquid securities may be particularly acute in situations in which the Fund’s operations require cash and could result in the Fund borrowing to meet its short-term needs or incurring losses on the sale of illiquid securities. See “Risk Factors — Fund Risks — Liquidity Risk.”

Convertible Securities Risk. The value of a convertible security is influenced by both the yield of non-convertible securities of comparable issuers and by the value of the underlying common stock. The value of a convertible security viewed without regard to its conversion feature (i.e., strictly on the basis of its yield) is sometimes referred to as its “investment value.” A convertible security’s investment value tends to decline as prevailing interest rate levels increase. Conversely, a convertible security’s investment value tends to increase as prevailing interest rate levels decline.

However, the convertible’s market value tends to reflect the market price of the common stock of the issuing company when that stock price is greater than the convertible’s “conversion price.” The conversion price is defined as the predetermined price at which the convertible could be exchanged for the associated stock. As the market price of the underlying common stock declines, the price of the convertible security tends to be influenced more by the yield of the convertible security and changes in interest rates. Thus, the convertible security may not decline in price to the same extent as the underlying common stock. In the event of a liquidation of the issuing company, holders of convertible securities would be paid before the company’s common stockholders. See “Risk Factors — Fund Risks — Convertible Securities Risk.”

Synthetic Convertible Instruments Risk. The value of a synthetic convertible instrument may respond differently to market fluctuations than a convertible security because a synthetic convertible instrument is composed of two or more separate instruments, each with its own market value. In addition, if the value of the underlying common stock or the level of the index involved in the convertible component falls below the exercise price of the warrant or option, the warrant or option may lose all value. See “Risk Factors — Fund Risks — Synthetic Convertible Instruments Risk.”

Risks Associated with Options. There are several risks associated with transactions in options. For example, there are significant differences between the securities markets and options markets that could result in an imperfect correlation among these markets, causing a given transaction not to achieve its objectives. A decision as to whether, when and how to use options involves the exercise of skill and judgment, and even a well-conceived transaction may be unsuccessful to some degree because of market behavior or unexpected events. The Fund’s ability to utilize options successfully will depend on Calamos’ ability to predict pertinent market movements, which cannot be assured.

The Fund may sell options on individual securities and securities indices. All calls sold by the Fund must be “covered.” Even though the Fund will receive the option premium to help protect it against loss, a call option sold by the Fund exposes the Fund during the term of the option to possible loss of opportunity to realize appreciation in the market price of the underlying security or instrument and may require the Fund to hold a security or instrument that it might otherwise have sold. In addition, a loss on a call option sold may be greater than the premium received. See “Risk Factors — Fund Risks — Risks Associated with Options.”

Forward Currency Exchange Contracts Risk. Forward contracts are contractual agreements to purchase or sell a specified currency at a specified future date (or within a specified time period) at a price set at the time of the contract. The Fund may not fully benefit from, or may lose money on, forward currency exchange transactions if changes in currency exchange rates do not occur as anticipated or do not correspond accurately to changes in the value of the Fund’s holdings.

Tax Risk. The Fund may invest in certain securities, such as certain convertible and high yield securities, for which the federal income tax treatment may not be clear or may be subject to re-characterization by the Internal Revenue Service (“IRS”). It could be more difficult for the Fund to comply with the federal income tax requirements applicable to regulated investment companies if the tax characterization of the Fund’s investments is not clear or if the tax treatment of the income from such investments was successfully challenged by the IRS. See “Risk Factors — Fund Risks — Tax Risk” and “Certain Federal Income Tax Matters.”

Management Risk. Calamos’ judgment about the attractiveness, relative value or potential appreciation of a particular sector, security or investment strategy may prove to be incorrect. See “Risk Factors — Fund Risks — Management Risk.”

Antitakeover Provisions. The Fund’s Agreement and Declaration of Trust and By-Laws include provisions that could limit the ability of other entities or persons to acquire control of the Fund or to change the composition of its Board of Trustees. Such provisions could limit the ability of shareholders to sell their shares at a premium over prevailing market prices by discouraging a third party from seeking to obtain control of the Fund. These provisions include staggered terms of office for the Trustees, advance notice requirements for shareholder proposals, and super-majority voting requirements for certain transactions with affiliates, converting the Fund to an open-end investment company or a merger, asset sale or similar transaction. Holders of preferred shares, if any, may have voting rights in addition to and separate from the voting rights of common shareholders with respect to certain of these matters. Holders of any preferred shares, voting separately as a single class, have the right to elect at least two Trustees at all times. See “Description of Securities — Preferred Shares” and “Certain Provisions of the Agreement and Declaration of Trust and By-Laws, Including Antitakeover Provisions.” The holders of preferred shares or debt, if any, on the one hand, and the holders of the common shares, on the other, may have interests that conflict with each other in other situations, including conflicts that relate to the fees and expenses of the Fund. For more information on potential conflicts of interest between holders of common shares and holders of preferred shares, see “Fund Risks — Leverage Risk.” See also “Risk Factors — Fund Risks — Antitakeover Provisions.”

Market Disruption Risk. Certain events have a disruptive effect on the securities markets, such as terrorist attacks, war and other geopolitical events, earthquakes, storms and other disasters. The Fund cannot predict the effects of similar events in the future on the U.S. economy or any foreign economy. See “Risk Factors — Fund Risks — Market Disruption Risk.”

Counterparty and Settlement Risk. Trading options, futures contracts, swaps and other derivative financial instruments entails credit risk with respect to the counterparties. Such instruments when traded over the counter do not include the same protections as may apply to trading derivatives on organized exchanges. Substantial losses may arise from the insolvency, bankruptcy or default of a counterparty and risk of settlement default of parties with whom it trades securities. This risk may be heightened during volatile market conditions. Settlement mechanisms in emerging markets are generally less developed and reliable than those in more developed countries thus increasing the risks. In the past, broker-dealers and other financial institutions have experienced extreme financial difficulty, sometimes resulting in bankruptcy of the institution. Although Calamos monitors the creditworthiness of the Fund’s counterparties, there can be no assurance that the Fund’s counterparties will not experience similar difficulties, possibly resulting in losses to the Fund. If a counterparty becomes bankrupt, or otherwise fails to perform its obligations under a derivative contract due to financial difficulties, the Fund may experience significant delays in obtaining any recovery under the derivative contract in a bankruptcy or other reorganization proceeding. The Fund may obtain only a limited recovery or may obtain no recovery in such circumstances. Material exposure to a single or small group of counterparties increases the Fund’s counterparty risk. See “Risk Factors — Fund Risks — Counterparty and Settlement Risk.”

Recent Market Events. In the past decade, financial markets throughout the world have experienced increased volatility, depressed valuations, decreased liquidity and heightened uncertainty and turmoil. This turmoil resulted in unusual and extreme volatility in the equity and debt markets, in the prices of individual securities and in the world economy. Events that have contributed to these market conditions include, but are not limited to, major cybersecurity events, geopolitical events (including wars and terror attacks), measures to address budget deficits, downgrading of sovereign debt, declines in oil and commodity prices, dramatic changes in currency exchange rates, and public sentiment. In addition, many governments and quasigovernmental entities throughout the world have responded to the turmoil with a variety of significant fiscal and monetary policy changes, including, but not limited to, direct capital infusions into companies, new monetary programs and dramatically lower interest rates.

Following the financial crisis that began in 2007, the Federal Reserve attempted to stabilize the U.S. economy and support the U.S. economic recovery by keeping the federal funds rate low. More recently, the Federal Reserve has terminated certain of its market support activities and began raising interest rates. The withdrawal of this support could negatively affect financial markets generally as well as reduce the value and liquidity of certain securities. Additionally, with continued economic recovery and the cessation of certain

market support activities, the Portfolio may face a heightened level of interest rate risk as a result of a rise or increased volatility in interest rates. These policy changes may reduce liquidity for certain of the Portfolio’s investments, causing the value of the Portfolio’s investments and share price to decline. To the extent the Portfolio experiences high redemptions because of these policy changes, the Portfolio may experience increased portfolio turnover, which will increase the costs that the Portfolio incurs and may lower the Portfolio’s performance.

Continuing uncertainty as to the status of the Euro and the European Monetary Union (“EMU”) and the potential for certain countries to withdraw from the institution has created significant volatility in currency and financial markets generally. Any partial or complete dissolution of the EMU could have significant adverse effects on currency and financial markets, and on the values of a Fund’s portfolio investments. In June 2016, the United Kingdom approved a referendum to leave the European Union (“EU”).

On March 29, 2017, the United Kingdom formally notified the European Council of its intention to leave the EU. As a result, the United Kingdom will remain a member state, subject to European law, with privileges to provide services under the single market directives for at least two years from that date. Given the size and importance of the United Kingdom’s economy, uncertainty about its legal, political, and economic relationship with the remaining member states of the EU may continue to be a source of instability. Moreover, other countries may seek to withdraw from the European Union and/or abandon the euro, the common currency of the EU. A number of countries in Europe have suffered terror attacks, and additional attacks may occur in the future. The Ukraine has experienced ongoing military conflict; this conflict may expand and military attacks could occur elsewhere in Europe. Europe has also been struggling with mass migration from the Middle East and Africa. The ultimate effects of these events and other socio-political or geographical issues are not known but could profoundly affect global economies and markets.

As a result of political and military actions undertaken by Russia, the U.S. and the EU have instituted sanctions against certain Russian officials and companies. These sanctions and any additional sanctions or other intergovernmental actions that may be undertaken against Russia in the future may result in the devaluation of Russian currency, a downgrade in the country’s credit rating, and a decline in the value and liquidity of Russian securities. Such actions could result in a freeze of Russian securities, impairing the ability of a fund to buy, sell, receive, or deliver those securities. Retaliatory action by the Russian government could involve the seizure of US and/or European residents’ assets, and any such actions are likely to impair the value and liquidity of such assets. Any or all of these potential results could have an adverse/recessionary effect on Russia’s economy. All of these factors could have a negative effect on the performance of funds that have significant exposure to Russia.

In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. See “Risk Factors — Fund Risks — Recent Market Events.”

Additional Risks to Common Shareholders

Additional risks of investing in common shares include the following:

Interest Rate Transactions Risk. The Fund may enter into an interest rate swap or cap transaction to attempt to protect itself from increasing dividend or interest expenses on its leverage resulting from increasing short-term interest rates. A decline in interest rates may result in a decline in the value of the swap or cap, which may result in a decline in the net asset value of the Fund. See “Risk Factors — Interest Rate Transactions Risk.”

Reduction of Leverage Risk. We have previously taken, and may in the future take, action to reduce the amount of leverage employed by the Fund. Reduction of the leverage employed by the Fund, including by redemption of preferred shares, will in turn reduce the amount of assets available for investment in portfolio securities. This reduction in leverage may negatively impact our financial performance, including our ability to sustain current levels of distributions on common shares.

Market Impact Risk. The sale of our common shares (or the perception that such sales may occur) may have an adverse effect on prices in the secondary market for our common shares. An increase in the number of common shares available may put downward pressure on the market price for our common shares. These sales also might make it more difficult for us to sell additional equity securities in the future at a time and price we deem appropriate.

Diminished Voting Power and Excess Cash Risk. The voting power of current shareholders will be diluted to the extent that such shareholders do not purchase shares in any future common share offerings or do not purchase sufficient shares to maintain their percentage interest. In addition, if we are unable to invest the proceeds of such offering as intended, our per share distribution may decrease (or may consist of return of capital) and we may not participate in market advances to the same extent as if such proceeds were fully invested as planned.

Market Discount Risk. The Fund’s common shares have traded both at a premium and at a discount relative to net asset value. Common shares of closed-end investment companies frequently trade at prices lower than their net asset value. Depending on the premium of the Fund’s common shares, the Fund’s net asset value may be reduced immediately following an offering of the Fund’s common shares by the offering expenses paid by the Fund. See “Use of Proceeds.”

In addition to net asset value, the market price of the Fund’s common shares may be affected by such factors as the Fund’s use of leverage, dividend stability, portfolio credit quality, liquidity, market supply and demand of the common shares and the Fund’s dividends paid (which are, in turn, affected by expenses), call protection for portfolio securities and interest rate movements. See “Leverage,” “Risk Factors” and “Description of Securities.” The Fund’s common shares are designed primarily for long-term investors, and you should not purchase common shares if you intend to sell them shortly after purchase.

See “Risk Factors — Additional Risks to Common Shareholders” for a more detailed discussion of these risks.

Additional Risks to Senior Security Holders

Additional risks of investing in senior securities include the following:

Generally, an investment in preferred shares (including exchange-listed preferred shares) or debt securities (collectively, “senior securities”) is subject to the following risks:

Interest Rate Risk. Rising market interest rates could impact negatively the value of our investment portfolio, reducing the amount of assets serving as asset coverage for the senior securities. Rising market interest rates could also reduce the value of the Fund’s senior securities.

Senior Leverage Risk. Preferred shares will be junior in liquidation and with respect to distribution rights to debt securities and any other borrowings. Senior securities representing indebtedness may constitute a substantial lien and burden on preferred shares by reason of their prior claim against our income and against our net assets in liquidation. We may not be permitted to declare dividends or other distributions with respect to any series of preferred shares unless at such time we meet applicable asset coverage requirements and the payment of principal or interest is not in default with respect to any borrowings.

Ratings and Asset Coverage Risk. To the extent that senior securities are rated, a rating does not eliminate or necessarily mitigate the risks of investing in our senior securities, and a rating may not fully or accurately reflect all of the credit and market risks associated with that senior security. A rating agency could downgrade the rating of our shares of preferred stock or debt securities, which may make such securities less liquid in the secondary market, though potentially with higher resulting interest rates. If a rating agency downgrades the rating assigned to a senior security, we may alter our portfolio or redeem the senior security. We may voluntarily redeem senior securities under certain circumstances.

Inflation Risk. Inflation is the reduction in the purchasing power of money resulting from an increase in the price of goods and services. Inflation risk is the risk that the inflation adjusted or “real” value of an investment in preferred stock or debt securities or the income from that investment will be worth less in the future. As inflation occurs, the real value of the preferred stock or debt securities and the dividend payable to holders of preferred stock or interest payable to holders of debt securities declines.

Decline in Net Asset Value Risk. A material decline in our NAV may impair our ability to maintain required levels of asset coverage for any preferred securities or debt securities we may issue in the future.

Secondary Market Risk. The market value of exchange-listed preferred shares that the Fund may issue will be determined by factors such as the relative demand for and supply of the preferred shares in the market, general market conditions and other factors beyond the control of the Fund. It may be difficult to predict the trading patterns of preferred shares, including the effective costs of trading. There is a risk that the market for preferred shares may be thinly traded and relatively illiquid compared to the market for other types of securities.

Market Discount Risk. The market price of exchange-listed preferred shares that the Fund may issue may also be affected by such factors as the Fund’s use of leverage, dividend stability, portfolio credit quality, liquidity, and the Fund’s dividends paid (which are, in turn, affected by expenses), call protection for portfolio securities and interest rate movements.

Early Redemption Risk. The Fund may voluntarily redeem preferred shares or may be forced to redeem preferred shares to meet regulatory requirements and the asset coverage requirements of the preferred shares. Such redemptions may be at a time that is unfavorable to holders of the preferred shares.

See “Risk Factors — Additional Risks to Senior Security Holders” for a more detailed discussion of these risks.

SUMMARY OF FUND EXPENSES

The following table and example contain information about the costs and expenses that common shareholders will bear directly or indirectly. In accordance with Commission requirements, the table below shows our expenses, including interest payments on borrowed funds, and preferred stock dividend payments, as a percentage of our average net assets as of April 30, 2018, and not as a percentage of gross assets or managed assets.

By showing expenses as a percentage of average net assets, expenses are not expressed as a percentage of all of the assets we invest. The table and example are based on our capital structure as of April 30, 2018. As of April 30, 2018, we had $25 million in borrowings outstanding, $12 million in outstanding preferred shares and additional structural leverage of $18 million, collectively representing 33.2% of managed assets as of that date.

Example:

The following example illustrates the expenses that common shareholders would pay on a $1,000 investment in common shares, assuming (1) total annual expenses of 2.80% of net assets attributable to common shareholders; (2) a 5% annual return; and (3) all distributions are reinvested at net asset value:

The example should not be considered a representation of future expenses. Actual expenses may be greater or less than those assumed. Moreover, our actual rate of return may be greater or less than the hypothetical 5% return shown in the example.

(1)If the securities to which this prospectus relates are sold to or through underwriters, the prospectus supplement will set forth any applicable sales load and the estimated offering expenses borne by us.

(2) Shareholders will pay a $15.00 transaction fee plus a $0.02 per share brokerage charge if they direct the Plan Agent to sell common shares held in a Plan account. In addition, each participant will pay a pro rata share of brokerage commissions incurred with respect to the Plan Agent’s open-market purchases in connection with the reinvestment of dividends or distributions. If a participant elects to have the Plan Agent sell part or all of his or her common shares and remit the proceeds, such participant will be charged his or her pro rata share of brokerage commissions on the shares sold. See “Dividends and Distributions on Common Shares; Automatic Dividend Reinvestment Plan.”

(3) The Fund pays Calamos an annual management fee, payable monthly, for its investment management services in an amount equal to 1.00% of the Fund’s average weekly managed assets. In accordance with the requirements of the Commission, the table above shows the Fund’s management fee as a percentage of average net assets attributable to common shareholders. By showing the management fee as a percentage of net assets, the management fee is not expressed as a percentage of all of the assets the Fund intends to invest. For purposes of the table, the management fee has been converted to 1.46% of the Fund’s average weekly net assets as of April 30, 2018 by dividing the total dollar amount of the management fee by the Fund’s average weekly net assets (managed assets less outstanding leverage).

(4) Reflects interest expense paid on $25.9 million in average borrowings under the SSB Agreement, plus $14.6 million in additional average structural leverage related to certain securities lending programs, as described under “Leverage.”

(5)Reflects estimated dividend expense on $12.0 million aggregate liquidation preference of mandatory redeemable preferred shares outstanding. See “Leverage.”

(6)“Other Expenses” are based on estimated amounts for the current fiscal year.

(7)The example does not include sales load or estimated offering costs, which would cause the expenses shown in the example to increase. In connection with an offering of common shares, the applicable prospectus supplement will set forth an example including sales load and estimated offering costs.

The purpose of the table and the example above is to help investors understand the fees and expenses that they, as common shareholders, would bear directly or indirectly. For additional information with respect to our expenses, see “Management of the Fund.”

FINANCIAL HIGHLIGHTS

The information in the following table shows selected data for a common share outstanding throughout each period listed below. The information in this table for the year ended October 31, 2017 and each of the prior years then ended is derived from our financial statements audited by Deloitte & Touche LLP, whose report on such financial statements is contained in our annual report dated October 31, 2017 and is included in the statement of additional information, both of which are available from us. See “Available Information” in this prospectus. The information shown for the six months ended April 30, 2018 is unaudited.

Selected data for a share outstanding throughout each period were as follows: