Official IRS Government site detailing this scam and providing links to report it:
http://www.irs.gov/uac/Newsroom/IRS-Warns-of-Pervasive-Telephone-Scam
If you feel you SSN has been compromised, in addition to putting a fraud alert at the 3 credit bureaus, you may want to notify the IRS. Here is the link, fill out the proper form (probably 14039):
http://www.irs.gov/Individuals/How-Do-You-Rep ... aud-Activity%3F