why my voice breaks up when on a phone call

Hi,

Usually, this issue doesn’t come directly from your iPhone, but your network (= your carrier)

So I actually advise you to make sure you’re in a good zone with a good network (check network status bar)

You can also enable VoLTE for your calls to improve voice quality in settings/cellular/data options/Voice & Data/enable VoLTE

Please note not all carriers offer this option.