Network Operations Expenses. Network operations expenses include the costs of personnel associated with service delivery, network management and customer support, network facilities costs, fiber and equipment maintenance fees, leased circuit costs, access and facilities fees paid to building owners and excise taxes billed to our customers and recorded on a gross basis. Our network operations expenses, including non-cash equity-based compensation expense, increased by 3.9% for the three months ended September 30, 2021 from the three months ended September 30, 2020. The increase in network operations expense is primarily attributable to an increase in costs related to our network and facilities expansion activities and an increase in taxes billed to our customers recorded on a gross basis.
Selling, General, and Administrative (“SG&A”) Expenses. Our SG&A expenses, including non-cash equity-based compensation expense, increased by 1.0% for the three months ended September 30, 2021 from the three months ended September 30, 2020. 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 $6.4 million for the three months ended September 30, 2021 and $6.2 million for the three months ended September 30, 2020. SG&A expenses increased primarily from an increase in salaries and related costs required to support our expansion related activities. Our sales force headcount decreased from 740 at September 30, 2020 to 662 at September 30, 2021, and our total headcount decreased from 1,110 at September 30, 2020 to 1,031 at September 30, 2021. We experienced an increase in both voluntary and involuntary employee departures, particularly within our sales department, in the three months ended September 30, 2021. We believe this rise in departures is attributable both to an increased focus on monitoring sales productivity and to the unwillingness of some employees to be vaccinated and/or to return to a full time, in office environment.
Depreciation and Amortization Expenses. Our depreciation and amortization expense increased by 4.6% for the three months ended September 30, 2021 from the three months ended September 30, 2020. The increase is primarily due to the depreciation expense associated with the increase in deployed fixed assets.
Interest Expense and Losses on Debt Extinguishment and Redemption. Our interest expense resulted from interest incurred on our senior secured notes due 2022 (“2022 Notes”) until these notes were fully redeemed in May 2021, interest incurred on our €350.0 million senior unsecured notes due 2024 (“2024 Notes”), interest incurred on our $500.0 million senior secured notes due in 2026 (“2026 Notes”), interest incurred on our installment payment agreement and interest incurred on our finance lease obligations. We issued €215.0 million of our 2024 Notes in June 2020 and €135.0 million of our 2024 Notes were issued in June 2019.In March 2021, we redeemed and extinguished $115.9 million of our 2022 Notes at 103.24% of par value. In May 2021, we redeemed and extinguished the remaining $329.1 million of our 2022 Notes at par value and deposited funds with the trustee to pay $11.5 million of interest through December 1, 2021. In May 2021, we issued $500.0 million of our 3.50% 2026 Notes.
In August 2021 we entered into an interest rate swap agreement (the “Swap Agreement”) that has the economic effect of modifying the fixed interest rate obligation associated with our 2026 Notes to a variable interest rate obligation based on the Secured Overnight Financing Rate (“SOFR”) so that the interest payable on the 2026 Notes effectively became variable based on overnight SOFR. The Swap Agreement is recorded at its fair value at each reporting period, and we incurs gains and losses due to changes in market interest rates. The values that we report for the Swap Agreement as of each reporting date are recognized as interest expense with the corresponding amounts included in assets or liabilities in the our consolidated balance sheets. As of September 30, 2021 the fair value of the Swap Agreement was a long-term liability $3.1 million and we recorded an unrealized loss as interest expense related to the Swap Agreement of $3.1 million in the three months ended September 30, 2021. Our interest expense increased by 10.1% for the three months ended September 30, 2020 to the three months ended September 30, 2021 primarily due to the lower interest rate on our 2026 Notes as compared to our 2022 Notes that we extinguished being offset by the additional $3.1 million of unrealized loss recorded as interest expense related to our Swap Agreement.
Unrealized gain (loss) on foreign exchange – 2024 Notes. Our 2024 Notes were issued in Euros and are reported in our reporting currency – US Dollars. As of September 30, 2021, our 2024 Notes were valued at $405.6 million. Our unrealized gain (loss) on foreign exchange on our 2024 Notes from converting our 2024 Notes into USD was $10.2 million for the three months ended September 30, 2021 and $(17.3) million for the three months ended September 30, 2020. We do not enter into hedges for our foreign currency obligations.
Income Tax (Provision) Benefit. Our income tax provision was $8.7 million for the three months ended September 30, 2021, and our income tax benefit was $1.6 million for the three months ended September 30, 2020. The increase in our income tax provision is primarily related to the increase in our income before income taxes.
Buildings On-net. As of September 30, 2021 and 2020, we had a total of 3,008 and 2,884 on-net buildings connected to our network, respectively. The increase in our 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.