Scammers pretending to be federal investigators for the IRS. Pretending to have a warrant.
IRS will ALWAYS contact you by mail first.
String the [***] along, see how long it takes before they figure out you know the scam and hang up.
All you are doing is encouraging retribution from the scammers. You will receive more calls from them, as well as other scammers. Later on, you will complain about the volume of scam calls that you receive, and moaning about how the government is not doing anything about it.