First, Make Sure It’s Your iPhone’s Fault

Remember that you need at least two phones to make a call. If everything seems fine on your end, it might be the other person’s fault.

The quickest way to figure this out is to call someone else and listen to the sound quality of that call. If the new call sounds the same or better on your end, the other person might have issues with their phone. If possible, ask them to call someone else, too, to see if they’re still having problems with their call.