iphone 13 garbled calls

iPhone 13 / almost every second call i make is garbled, voice is unrecognisable. have to call again, if im lucky i can hear the voice or i reboot phone

re booted phone / same

factory reset phone / same

put sim card in old iphone 6s / works perfectly

put sim card in cheap doro phone (30 pounds) / works perfectly

phone has never been dropped.

cost of phone 800 pounds / worst product ive ever bought