Hi
I had this problem, and although some people say it's the Noise Cancellation setting (General - Accessibility - Hearing), which did make a small difference, I found what was really causing the problem on my phone.
I have a case where the front of the phone is covered by a flap, and I was folding the flap back when receiving and making calls. If I stopped folding the flap right back and just held it open, the call quality improved immediately.
I hope that helps.