The phone number for the Internal Revenue Service is 1-800-829-1040. This is the toll-free number for individual taxpayers. Tax professionals must use 1-866-255-0654 instead. For those outside of the United States, the number to call is 1-512-416-7750.
The number for individuals is the IRS customer service number. The lines for that number are open Monday though Friday from 7:00 a.m. to 7:00 p.m. based on the caller's local time. Since the IRS is a government organization, the lines are closed on federal holidays. The number for tax professionals is available Monday through Friday from 6:30 a.m. to 6:00 p.m. Central Time.