They are not legit. They actually say they are with the US Treasury Department. However someone from within the IRS, a clerk perhaps, must be illegally providing a list of names of people who may at one time had some discrepancy, to these people. They are using scare tactics (a warrant will be issued and a Sheriff is on his way to your place of residence, etc.) but don't fall for them. Tell them that you work for the IRS and they hang up. Tell them you'll be home for the cops when they come, and they hang up in frustration.
It is probably a computer dialed robocall that dials numbers in order. The call before you was one number lower & the one afterwards was one number higher. Note they do not tell who the call is for.