The IRS does not call you out of the blue. Medicare does not call you out of the blue. Your electric company does not call to say the truck is on its way to shut off your power in 30 minutes unless you pay over the phone. These institutions move slowly and they move in writing. If there is a real problem with your taxes, your benefits, or your account, a letter shows up first. Often several letters, over weeks.
A phone call that skips the letter step and jumps straight to urgency is the tell. The caller is hoping you do not know the normal sequence. Now you do. Hang up, and if you want to confirm, call the agency or utility back through a number you look up yourself.