If you mailed your federal return and are expecting a refund you can start checking the status of your return on the IRS website Where's My Refund 4 weeks after mailing your return. It takes that amount of time for it to show up in their system.
You only want to call them if it’s been at least 6 weeks since you mailed and at that time, you can reach them at the IRS tax refund hotline at 800-829-4477, Monday - Friday, 7:00 a.m. to 11:30 p.m.