This is NOT the IRS. The IRS will never call you to initiate a communication; under any circumstances. This number is used as a callback number for a scam. Under no circumstances should you give them any information or follow any of their instructions. Simply hang up.
All legitimate initial communications from the IRS will be received either in-person or through registered postal mail.
For more information about this scam, see: http://www.irs.gov/uac/Tax-Scams-Consumer-Alerts . You can also visit http://www.irs.gov/uac/Report-Phishing for instructions on what to do if you receive a call claiming to be from the IRS. You can call, toll-free, 1-800-366-4484 to validate any IRS communication and to see if the IRS has a legitimate reason to contact you. This number is provided on the irs.gov website; linked above. Caller ID information can be spoofed, so if you receive a call from the toll-free number, you should call it back to verify that it is legitimate.