Experience and Foresight

About Calamos Investments

For nearly 35 years, we have helped investors like you manage and build wealth to meet their long-term individual objectives by working to capitalize on the opportunities of the evolving global marketplace. We launched our first open-end mutual fund in 1985 and our first closed-end fund in 2002. Today, we manage five closed-end funds. Three are enhanced fixed income offerings, which pursue high current income from income and capital gains. Two are income-oriented total return offerings, which seek current income, with increased emphasis on capital gains potential. Calamos Global Total Return Fund (CGO), falls into this category. Please see page 5 for a more detailed overview of our closed-end offerings.

We are dedicated to helping our clients build and protect wealth. We understand when you entrust us with your assets, you also entrust us with your achievements, goals and aspirations. We believe we best honor this trust by making investment decisions guided by integrity, by discipline, and by our conscientious research.

We believe that an active, risk-conscious approach is essential for wealth creation. In the 1970s, we pioneered strategies that seek to participate in equity market upside and mitigate some of the potential risks of equity market volatility. Our investment process seeks to manage risk at multiple levels and draws upon our experience investing through multiple market cycles.

We have a global perspective. We believe that globalization offers tremendous opportunities for countries and companies all over the world. In our view, this creates significant opportunities for investors. In our U.S., global and international portfolios, we are seeking to capitalize on the potential growth of the global economy.

We believe there are opportunities in all markets. Our history traces back to the 1970s, a period of significant volatility and economic concerns. We have invested through multiple market cycles, each with its own challenges. Out of this experience comes our belief that the flipside of volatility is opportunity.

JOHN P. CALAMOS, SR.

CEO and Global Co-CIO

Dear Fellow Shareholder:

Welcome to your annual report for the period ended October 31, 2012. This report includes commentary from our investment team, as well as a listing of portfolio holdings, financial statements and highlights, and detailed information about the performance and allocation of your fund. I invite you to read it carefully.

Calamos Global Total Return Fund (CGO) is an income-oriented total return fund. This means we are focused not only on delivering a competitive stream of distributions, but also on total return.

Steady and Competitive Distributions

During the annual period, CGO provided steady monthly distributions. We believe the Fund’s distribution rate, which was 8.88% on a market price basis as of October 31, 2012, was very competitive, given the low interest rates in many segments of the bond market. In our view, the Fund’s distributions illustrate the benefits of a global, multi-asset class approach and flexible allocation strategy.

We understand that many closed-end fund investors seek steady, predictable distributions instead of distributions that fluctuate. Therefore, this Fund has a level rate distribution policy. As part of this policy, we aim to keep distributions consistent from month to month, and at a level that we believe can be sustained over the long term. In setting the Fund’s distribution rate, the investment management team and the Fund’s Board of Trustees consider the interest rate, market and economic environment. We also factor in our assessment of individual securities and asset classes. (For additional information on our level rate distribution policy, please see “The Calamos Closed-End Funds: An Overview” on page 5 and “Level Rate Distribution Policy” on page 32.)

Market Environment

Global markets advanced impressively over the 12-month period, even as stop-and-go rallies and persistent volatility combined to keep investors in a heightened state of uncertainty. For every positive development like strong corporate earnings, an improving U.S. housing market and the ongoing liquidity measures of the world’s central banks, the markets were given pause by continued economic instability in Europe, slowing growth in China and a still lackluster U.S. employment market. As the period ended, investors were also confronted with the uncertainty of the pending fiscal cliff.

Overall for the 12-month period, however, investors seemed to focus more on the positives than the negatives, as the global bond and equity markets posted strong performance. The BofA Merrill Lynch Global Broad Market Index returned 3.75%, while the Credit Suisse High Yield Index returned 12.86%. Many segments of the global equity market also advanced. The MSCI World Index, a measure of developed market equity performance returned 10.11%. The U.S. equity market performed even better, with the S&P 500 Index up 15.21%, and emerging market equities continued to climb back with a 2.98% gain in the MSCI Emerging Markets Index.

Outlook: Slow Growth Amid Uncertainty

We believe that the U.S. economy is positioned to stay on its slow-growth course. Despite the significant challenges of the fiscal cliff and the ever-changing situation in the euro zone, consumers have remained relatively resilient and some manufacturing data, including a healthier auto industry, is pointing in a better direction. Businesses have improved their balance sheets and have plenty of cash on hand, ready to invest and grow in the coming year. The recovery of the housing market, however gradual, can also provide a boost to the economy as rising prices contribute to increased consumer confidence.

Nonetheless, we are also cautious. We expect market volatility to continue, stoked in part by a range of near-term political uncertainties, including the recent leadership change in China and ongoing partisan contentiousness in the U.S. While quantitative easing has proven to an effective short-term balm for the markets, the long-term effectiveness of the government continuing to pump money into the economy is far less assured. In the U.S., job growth continues to be uninspiring, and while money supply has soared, the velocity of money has not. Although large companies have benefitted from low rates and have accumulated capital, smaller businesses face persistent hurdles as banks keep the purse strings tight.

In terms of global growth, emerging markets remain a driving force. While we are closely watching China, particularly to see how the government responds to softening consumer demand within its borders, growth remains strong compared to many other economies. We believe the secular trends related to the emerging markets, such as the rise in consumer income and increased middle class consumption, can help China and other emerging markets through inevitable periods of slower growth.

The situation in Europe, however, is not nearly as encouraging. While intervention from the European Central Bank provided some short-term solvency relief to the beleaguered euro zone during the latter half of the recent 12-month period, the region’s long-term debt issues may continue to fester for quite some time. We expect volatility in Europe to remain high as countries like Spain and Greece continue to try and work through their problems, while their neighbors try to maintain patience and keep the region’s tenuous economic partnership intact.

We maintain our view that sustained global growth requires a greater commitment to pro-business policies and reduced government intervention. A strong private sector can set the stage for better wages and national prosperity, but rising government debt without rising personal income is a roadmap to economic stagnation.

In selecting investments for this Fund, we seek to balance yield and risk considerations. We therefore favor companies that we believe offer reliable debt servicing, respectable balance sheets and strong cash flows. As part of our active approach, we subject each investment to our rigorous fundamental credit research.

Our Use of Leverage

We have the flexibility to utilize leverage in this Fund. Over the long term, we believe that the judicious use of leverage provides us with opportunities to enhance total return and support the Fund’s distribution rate. Leverage strategies typically entail borrowing at short-term interest rates and investing the proceeds at higher rates of return. During the reporting period, we believed the prudent use of leverage would be advantageous given the economic environment, specifically the low borrowing costs we were able to secure. Overall, our use of leverage contributed favorably to the returns of the Fund, as the performance of the Fund’s holdings exceeded the costs of our borrowing activities.

Consistent with our focus on risk management, we have employed techniques to hedge against a rise in interest rates. We have used interest rate swaps to manage the borrowing costs associated with our leverage activities. Interest rate swaps allow us to “lock down” an interest rate we believe to be attractive. Although rates are at historically low levels across much of the fixed income market, history has taught us that rates can rise quickly, in some cases, in a matter of months. We believe that the Fund’s use of interest rate swaps is beneficial because it provides a degree of protection should a rise in rates occur.

Opportunities: Global Equities Remain Compelling

Reflecting our outlook for continued slower growth and our wariness about inflation, we maintain a constructive view on equities. We believe that global secular themes—including growing consumer strength in the emerging markets, global demand for technology innovations and global infrastructure build-out—can continue to power a wide range of companies across the globe, even against the headwind of slower-growth outlook.

Valuations continue to be attractive, particularly in this low interest rate environment. Moreover, there is money sitting on the sidelines that we believe could move into motion quickly, providing a tailwind to equities. In our opinion, the opportunities are most pronounced for multinational growth-oriented companies, particularly those poised to capitalize on the strengthening consumer power in the emerging markets. These companies have the flexibility to go where capital is treated best. Further, while interest rates remain historically low for now, equities can act as a hedge against the potential risk of inflation.

Within fixed income, we see opportunity in the mid-grade corporate sector. As yields in the government bond market have all but evaporated, this segment of the market continues to demonstrate attractive risk-reward characteristics for income-oriented investors. We are especially vigilant about interest rate risk, knowing from history how quickly inflation can sometimes arise.

The past year has been encouraging, but the sustainability of the markets’ positive performance is yet to be seen. This is an environment that requires patience and long-term, global perspective. We believe our dynamic allocation approach has been instrumental to the results we have achieved over full market cycles and will continue to be, particularly given the low rates currently available in many segments of the fixed income marketplace.

If you would like any additional information about this Fund or our other closed-end offerings, please contact your financial advisor or our client services team at 800.582.6959 (Monday through Friday from 8:00 a.m. to 6:00 p.m., Central Time), or visit us at calamos.com. We thank you for your continued trust.

Sincerely,

John P. Calamos, Sr.

CEO and Global Co-CIO,

Calamos Advisors LLC

This report is for informational purposes only and should not be considered investment advice.

GLOBAL TOTAL RETURN FUND

INVESTMENT TEAM DISCUSSION

The Fund’s Investment Team discusses strategy, performance and positioning for the 12-month period ended October 31, 2012.

Q.  To provide a context for its performance, please discuss the Fund’s strategy and role within an asset allocation.

A.  Calamos Global Total Return Fund (CGO) is a global income-oriented total return offering that seeks to provide an attractive monthly distribution. We invest in a diversified portfolio of global equities, convertible securities and high yield securities. The allocation to each asset class is dynamic and reflects our view of the economic landscape as well as the potential of individual securities. By combining these asset classes, we believe that we are well positioned to generate capital gains as well as income. This broader range of security types also provides us with increased opportunities to manage the risk and reward characteristics of the portfolio over full market cycles.

We invest in both U.S. and non-U.S. companies, favoring companies with geographically diversified revenue streams and global business strategies. We emphasize companies we believe offer reliable debt servicing, respectable balance sheets and sustainable prospects for growth.

Q.  How did the Fund perform over the reporting period?

A.  The Fund gained 4.55% on a net asset value (NAV) basis for the 12-month period ended October 31, 2012. On a market price basis, the Fund returned 0.29%. The Fund’s return did not perform as well as the broad global equity market, which was up 10.11% for the same period, as measured by the MSCI World Index.

Q.  How do NAV and market price return differ?

A.  Closed-end funds trade on exchanges, where the price of shares may be driven by factors other than the value of the underlying securities. The price of a share in the market is called market value. Market price may be influenced by factors unrelated to the performance of the fund’s holdings. A fund’s NAV return measures the return of the individual securities in the portfolio, less fund expenses. It also measures how a manager was able to capitalize on market opportunities. Because we believe closed-end funds are best utilized as a long-term holding within asset allocations, we believe that NAV return is the better measure of a fund’s performance.

Q.  Please discuss the Fund’s distributions during the annual period.

A.  We employ a level rate distribution policy within this Fund with the goal of providing shareholders with a consistent distribution stream. The Fund provided a steady distribution stream over the period. Monthly distributions were $0.1000 per share. The Fund’s annual distribution rate was 8.88% of market price as of October 31, 2012.

SINCE INCEPTION MARKET PRICE AND NAV HISTORY THROUGH 10/31/12

Performance data quoted represents past performance, which is no guarantee of future results. Current performance may be lower or higher than the performance quoted.

We believe that both the Fund’s distribution rate and level remained attractive and competitive, as low interest rates limited yield opportunities in much of the marketplace. For example, as of October 31, 2012, the dividend yield of S&P 500 Index stocks averaged 2.36%. Yields also remained low within the U.S. government bond market, with 10-year U.S. Treasurys and 30-year U.S. Treasurys yielding 1.72% and 2.85%, respectively.

Q.  The Fund is currently trading at a discount to its NAV. Please discuss this discount.

A.  As of the close of the reporting period, the Fund was trading at a discount of 3.22%. This means that its market share price is 3.22% less than its NAV price. At the beginning of the reporting period, the Fund was trading at a premium of 0.89%. As we have noted in the past, we believe that this may be favorable for long-term investors seeking to purchase shares because investors can buy shares of the portfolio at a price that is lower than the fair value of the portfolio, as measured by its NAV.

Q.  What factors influenced performance over the reporting period?

A.  Both convertibles and bonds provided income to the portfolio over the 12-month period, but in general convertibles did not provide as much equity upside as we would have preferred. Security selection within the energy sector was a drag on performance, as was an overweight position and selection within the materials sector, where our holdings within the metals and mining industry lagged.

An underweight position in financials also hindered returns. Given our concerns about the uncertain mortgage exposure on the balance sheets of many banks, coupled with the increasing costs of government regulation, we mostly eschewed this sector during the period. Security selection within the consumer discretionary and consumer staples sectors also hurt returns, while the Fund’s underweight to industrials did not fully optimize the returns to be had in the sector.

Security selection and an overweight position in the health care sector—the top-performing area in the MSCI World Index during the period—was additive to returns for the period. We continue to hold names in this area that we believe are best suited to capitalize on the world’s changing demographics. Although underweight in utilities, our selection enabled us to realize stronger-than-average returns in the sector for the period.

Fund asset allocations are based on total investments and may vary over time.

Q.  How is the Fund positioned?

A.  We continue to seek out securities in sectors that offer the best prospects for income-oriented total return. During the period, the Fund was overweight the health care, materials and information technology sectors, and underweight consumer discretionary, industrials and financials. Our focus continues to be on multinational companies that are able to participate in global growth opportunities, regardless of where they are domiciled. We expect these opportunities to occur in conjunction with a growing middle class in many of the developing economies across the globe.

The Fund is currently positioned with large absolute allocations to the information technology and health care sectors. We see firms within the technology sector as offering high growth prospects with strong cash flows and healthy balance sheets. With respect to health care, we view health care spending overall as typically more resilient in the context of low economic growth. Within the sector, we continue to favor select companies in the pharmaceutical and biotechnology industries, which offer a compelling combination of growth, high cash flows and solid fundamentals.

Relative to the MSCI World Index, the Fund’s largest absolute underweight allocation is to the financials sector. We have generally exercised caution in this area due to our expectation that increased regulations globally and persistent capital risks will decrease future returns on equity. We have marginally increased our exposure to the sector during the period, and continue to favor investment in globally diversified asset managers with visible revenues and relatively low underlying credit risk.

The average credit quality of the portfolio is higher than that of the index. This is typical for the Fund, as our credit process tends to guide us away from the most speculative corporate securities. We currently view the lowest credit tiers of the market as less attractive given their pricing and our outlook for a slower-growth global economy.

Q.  What is your outlook for the Fund?

A.  Our outlook for global growth equities remains favorable and we have positioned the Fund accordingly. In our view, equities continue to offer compelling risk-reward characteristics and we are finding companies with attractively valued fundamentals in many industries. Although we anticipate periods of heightened volatility, we also see significant growth potential in our secular themes related to innovation, mobility and productivity enhancement within technology, as well as the expanding middle class in emerging economies driving growth in consumer and health care areas, in particular. Additionally, continued global central bank reflation measures and infrastructure build-out are providing support to select companies in more cyclical sectors such as energy and materials, areas in which we have meaningful exposure.

We believe large multinational companies with globally diversified revenues and access to capital are poised to capitalize on both secular and cyclical trends. In our estimation, valuations of global equities are attractive, as investors continue to discount the growth of future cash flows. Equity and equity-sensitive securities have delivered solid gains during the last year but continue to offer attractive risk-reward characteristics from a long-term perspective. We continue to pursue our strategic objective to participate in a significant portion of equity market upside while aiming to reduce volatility versus global equities across a complete market cycle.

In regard to fixed income, we view the mid-grade credit space as particularly well-priced, offering both attractive levels of income with less exposure to potential inflation

and higher interest rates. In addition, the mid-grade credit space also offers far healthier companies than are available in the most speculative credit tiers. While the lowest-quality securities can offer higher yields, we find the healthier companies with improving credit profiles to be more suitable investments amid the current economic environment. As noted above, the Fund therefore maintains a higher average quality than the index as a result of this outlook.

The low interest rate environment continues to present a challenge to the convertible market. In the current environment, issuers are finding the low interest rates attractive without having to offer the equity enhancement. Our goal is to capture yield by focusing more on the credit sensitive side of the convertible market.

NOTES TO SCHEDULE OF INVESTMENTS

FOREIGN CURRENCY ABBREVIATIONS

Note: Value for securities denominated in foreign currencies is shown in U.S. dollars. The principal amount for such securities is shown in the respective foreign currency.

CURRENCY EXPOSURE

OCTOBER 31, 2012

Currency exposure may vary over time.