Total scam.  Originates in either India or Pakistan.
See:  http://www.irs.gov/uac/Newsroom/IRS-Reiterate ... -Telephone-Scam

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

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 has no powers of arrest.    If LE is necessary, US Marshals do the job.  IRS does not cold call anyone and ask for money. 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.