Can’t hear callers from iPhone 12pro

Another test, play some music and both ear and main speakers should sound equally loud. If not, it is busted or dislodged speaker.

Another sign is setting sound balance fully left has little effect.

This can happen on the12 if you have dropped it on its top end. Repairs at Apple would be approaching £€$500.

If the volume does not improve using the volume button during a call then damage is the most likely.