This is a Pakistan or India based IRS scam (there are also Indian based ones). Do not fall for this. Read http://www.treasury.gov/tigta/press/press_tigta-2014-03.htm
The IRS does not initiate any legal action via phone. Report it also to your local law enforcement agency. The scammers use voice over IP lines (e.g. magicjack (Ymax corporation). This is always against these companys' terms of use and can lead to termination of their contract. However only law enforcement can initiate the contact if you yourself are not a customer. Ultimately this is a case for the FBI legal attache in Islamabad or Delhi. If you are really annoyed: call or write to your representative in Congress. As these scams happen all over the country maybe this will trigger a reaction (and the NSA can finally be a force for good).