You get a call claiming to be from your phone provider, saying your account is in arrears and you need to pay now by card, ot you’ll be disconnected and it will cost you more to get reconnected.
If you’re with another provider the caller may well say that their company ownd the line, or you will have to pay a percentage to them for line rental.
If you protest they’ll offer to prove it by temporarily disconnecting you. The line will seem to go dead, until they call you back, but in fact they’ve simply used the mute button.