At this point there is not much you can do.  Invest in a call blocker.  These callers often have a lot of personal and financial information about you already.  You might consider changing out your various financial account numbers.

Total scam.  Originates in either India or Pakistan.   Block and ignore.   DO NOT RETURN THE CALL!
See:  http://www.irs.gov/uac/Newsroom/IRS-Reiterate ... -Telephone-Scam

Report them:
http://www.treasury.gov/tigta/contact_report_scam_ok.shtml

I’m an Enrolled Agent with the IRS (Inactive). IRS does not initiate contacts by phone.  US mail only.   IRS does not file lawsuits.   Court action, if any is handled by the DOJ and all parties are entitled to legal assistance if it is a criminal matter.  But--tax disputes are for the most part civil matters.  90% of all IRS tax disputes are settled administratively.  IRS has absolutely no authority to call on local law enforcement for anything. Local law is never involved in IRS tax disputes. IRS does not threaten to involve local law enforcement or to arrest you for nonpayment of federal taxes.  IRS has no powers of arrest.    If LE is necessary, US Marshals do the job.  IRS does not cold call anyone and demand immediate payment of  money.  IRS does not ask for financial information over the phone.   IRS does not accept Green card money paks for payment in any case.  No one is prosecuted for ordinary tax disputes. IRS has absolutely no authority over anyone's professional license. Professional licensing is handled by state governments.  Yes, levies on property to satisfy an existing tax obligation do occur, but only after a very long period of negotiation. And yes, liens are placed on individuals but  usually after a lot of negotiation and a failure to reach a settlement.    All determinations of tax liability are subject to appeals.  All  contacts by the IRS are initiated by US mail.