I do not answer "Unavailable" phone callers. Scammer trying to scare recipient with bogus "IRS TAX FRAUD CASE"
"Final Notice, call the following number immediately" What are you, twelve? Like any person not living in a cave would call you scammy people back. I reported the phone number to https://www.treasury.gov/tigta immediately. The more reports they get, the more ammunition they have with Congress to do something about these low-lifes.