This is a Pakistan or India based IRS scam. 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. 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. Therefore, report it also to your local law enforcement agency. Ultimately this is a case for the FBI legal attache in Islamabad or New 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).

You can use the following template for your letter or email:

Senators
Use the district office address of your senator as displayed on http://www.senate.gov/ 
Your letter should be addressed as follows:
The Honorable (full name)
United States Senate
Mailing Address of District Office
City, State, Zip Code

Representatives (House)
Use the district office address of your district representative as displayed on
http://www.house.gov/
Your letter should be addressed as follows:
The Honorable (full name)
United States House of Representatives
Mailing Address of District Office
City, State, Zip Code

Dear Senator/Representative XXX

As your constituent, I am writing to urge you to demand decisive action from the FBI, the IRS and other federal agencies against organized crime directed against American citizens from abroad.

As widely reported by local and national media all across the United States and warned about by the Treasury Inspector General for Tax Administration (see http://www.treasury.gov/tigta/press/press_tigta-2014-03.htm) a tidal wave of fraud using Internet phones has hit the United States. Millions of dollars already have been stolen by criminals impersonating federal officers while hiding in countries like India and Pakistan, where most of the calls originate. There seems to be no end of it in sight.

I have personally been targeted by these scammers but it is the most vulnerable among us that I am concerned about primarily. The scheme is specifically singling out those that can least defend themselves against these crimes, in particular the elderly. I am also worried that given the origin of these gangsters, the money stolen from US citizens may be used for financing of terrorism or other activities directed against the national interests of the United States. But in spite of this clear and present danger to our country and its citizens I have seen no action worth mentioning by the FBI, the IRS or other federal agencies so far.

As a member of Congress overseeing the law enforcement agencies tasked with fighting fraud and ensuring national security, I implore you to make sure that they act on these threats and are given all the necessary resources to target these criminals in cooperation with foreign law enforcement.

Yours sincerely