2019. Non-cash equity-based compensation expense is included in SG&A expenses consistent with the classification of the employee’s salary and other compensation and was $16.8 million for the nine months ended September 30, 2020 and $12.8 million for the nine months ended September 30, 2019. SG&A expenses increased primarily from an increase in salaries and related costs required to support our expansion and the increase in our sales efforts. Our sales force headcount increased from 667 at September 30, 2019 to 740 at September 30, 2020, our quota bearing sales force increased from 530 at September 30, 2019 to 597 at September 30, 2020 and our total headcount increased from 1,036 at September 30, 2019 to 1,110 at September 30, 2020.
Depreciation and Amortization Expenses. Our depreciation and amortization expense increased by 1.3% for the nine months ended September 30, 2020 from the nine months ended September 30, 2019. The increase is primarily due to an increase in our deployed fixed assets.
Interest Expense and Loss on Debt Extinguishment & Redemption. Our interest expense resulted from interest incurred on our $445.0 million of senior secured notes (“2022 Notes”), interest incurred on our $189.2 million of senior unsecured notes (“2021 Notes ") until redeemed in June 2020, interest on our installment payment agreement, interest on our finance lease obligations and interest on our €350.0 million ($410.4 million) aggregate principal amount of 4.375% senior unsecured notes (“2024 Notes’). We issued €215.0 million of our 2024 Notes in June 2020 and €135.0 million of our 2024 Notes in June 2019. We redeemed our 2021 Notes in June 2020 at par value resulting in a loss on debt extinguishment of $0.6 million in the nine months ended September 30, 2020. Our interest expense increased by 10.0% for the nine months ended September 30, 2020 from the nine months ended September 30, 2019 primarily due to an increase in our finance lease obligations and the issuances of our 2024 Notes partly offset by the redemption of our 2021 Notes.
Realized Gain and Unrealized Gain (Loss) on Foreign Exchange – 2024 Notes. In June 2020, we completed an offering of our €215.0 million principal amount of 2024 Notes. In June 2019, we completed an offering of our €135.0 million principal amount of 2024 Notes. In June 2020 our €215.0 million 2024 Notes were issued at a Euro to USD rate of $1.112. We received proceeds in USD on the 2024 Notes on June 9, 2020 at a Euro to USD rate of $1.133 resulting in a realized gain on foreign exchange of $2.5 million. Our 2024 Notes were issued in Euros and are reported in our reporting currency – US Dollars. As of September 30, 2020 the carrying value of our 2024 Notes was $410.4 million. Our unrealized (loss) gain on foreign exchange on our 2024 Notes from converting our 2024 Notes into USD was $(17.8) million for the nine months ended September 30, 2020 and $6.3 million for the nine months ended September 30, 2019. We do not enter into hedges for our foreign currency obligations.
Income Tax Provision. Our income tax provision was $4.7 million for the nine months ended September 30, 2020 and $11.9 million for the nine months ended September 30, 2019. The change in our income tax provision is primarily related to changes in our income before income taxes including the change in the unrealized (loss) gain on foreign exchange on our 2024 Notes.
Buildings On-net. As of September 30, 2020 and 2019, we had a total of 2,884 and 2,771 on-net buildings connected to our network, respectively. The increase in on-net buildings was a result of our disciplined network expansion program. We anticipate adding a similar number of buildings to our network for the next several years.
Liquidity and Capital Resources
As our business has grown as a result of an increasing customer base, broader geographic coverage and increased traffic, we have produced a growing level of operating cash flow. As a result of the operating leverage of our network, our annual capital expenditures as measured as a percentage of revenues has fallen over the last decade. We have also had increasing success in raising capital by issuing notes and arranging financing and leases that have had a lower cost and more flexible terms. The combination of this improved operating performance and access to capital has enhanced our financial flexibility and increased our ability to make distributions to shareholders in the form of cash dividends or through share repurchases. Since our initial public offering, we have returned over $858 million to shareholders through share repurchases and dividends. We intend on continuing to assess our capital and liquidity needs and where appropriate return capital to shareholders.
In assessing our liquidity, management reviews and analyzes our current cash balances, accounts receivable, accounts payable, accrued liabilities, capital expenditure commitments, and required finance lease and debt payments and other obligations.
We or our affiliates may, at any time and from time to time, seek to retire or purchase our outstanding debt through cash purchases and/or exchanges for equity or debt, in open-market purchases, privately negotiated transactions or otherwise. Such repurchases or exchanges, if any, will be upon such terms and at such prices as we may determine, and will depend on prevailing market conditions, our liquidity requirements, contractual restrictions and other factors. The amounts involved may be material.