Make sure you have your passport ready since you will need to fly to a country overseas. Of course once there if you are caught you are then subject to their laws. As to threatening them, they know it is a hollow threat and they have heard it all.
I have received several calls and voice mails from this same "Steve" saying that he was calling regarding enforcement actions for the US Treasury and that I could face a federal criminal offense and to call him back to avoid appearance in a federal court, blah blah. He usually calls early in the morning. One time I picked up and he knew my name; I told him to send me the information and not to call back. He has left two more voice mail messages, and I've had a couple of hang ups, too.
I got the same BS call, but it still scared me. I called back. "Pakistani Steve" left the message, but Robert Matthews answered and knew my name. Said I owed $8500 to the IRS, and there was a Federal arrest warrant for me. I told him I did owe money, but paid it. He said "I am sending the police to your house, you will be arrested in 45 minutes" and hung up. I am glad I found this page ..... I called them back multiple times and had my daughter scream into the phone with an ear piercing scream, then I filled out the form at the TIGTA website. A-holes!!!!