The voice is stern. There’s often a fake badge number and a fake case number. The amount owed is specific. You’re told not to hang up, not to call your lawyer, not to tell anyone, and to go buy gift cards or send a wire transfer in the next hour or the local police will arrest you today.
The IRS does not call you out of nowhere. The IRS does not accept gift cards. The IRS does not threaten arrest in a phone call. If you actually owe back taxes, the first thing you get is a letter, and the second thing is another letter, and you get to respond by mail.
This scam still works because the threat is paralyzing. People who would never wire money to a stranger will wire it when they think the alternative is jail. The script is engineered for that fear.