Tax fraud. Do not call back, just report it to TIGTA https://www.treasury.gov/tigta/contact_report_scam.shtml as they are actively attempting to prosecute these individuals.
I called them back using *67 to block my number. They answered and asked me who i was and i said i'm the guy who will kill you and your family with lasers on the heads of sharks. The guy didn't know what to say. So we all should call them back with *67 and harrass them too. Why should they only be the ones doing the harrassment?