Bob, these crooks are posing as IRS or Treasury Dept. "agents" and they try to make you shell out money (via credit card, money transfer even iTunes/Walmart gift cards!) They use threatening words like "lawsuit", "summons", "jail", "arrest" and so forth.
They are liars, cheats, frauds and crooks (not to mention scum of the earth.) Ignore the calls and DO NOT try to call them back. Better yet, get a call blocker (or blocker app for your smart phone) to help combat there heaping piles of poop.