The IRS telephone number is 1-800-829-1040, and they are available from 7 a.m. – 7 p.m. local time, Monday thru Friday.
You can expect a long wait when calling calling between May and December, with the longest waits typically on Mondays and Tuesdays.
The best times of day to call is before 9 a.m. on the East Coast and after 5 p.m. on the West Coast.
Alternatively, if you're unsuccessful reaching IRS by phone, you may file form 911 on IRS website or call Taxpayer Advocate local office at 877-777-4778.