Report it to TIGTA under http://www.treasury.gov/tigta/contact_report_scam.shtml especially if you have become a victim (lost money, gave personal information etc.). You can also file a complaint with the Federal Trade Commission at ftc.gov/complaint (include “IRS Telephone Scam" in your complaint). This way you can also help the government establish specific fraud patterns. (also read the above posts!)
Claim to be from IRS and contact them immediately before they take any action against.
When I called back the men with Indian or Pakistani accent answered that it's IRS and gave long ID number.