Disappointing Service

Good Day, This is directed to MTN Insurance. I received a Huawei Nova phone as replacement for my lost Huawei P40 lite in March this year. When I'm on calls with people, they can't hear me. They keep saying I'm breaking. I don't know what is the way forward because I first thought it might be due to loadshedding but same issue kept happening when it was not loadshedding. I logged a call with MTN and they took the phone in for repairs twice. It stays with MTN technical for 2 weeks at a time but the problem still persists. I am still paying for the contract. MTN keeps calling me to upgrade but how can I if they cannot sort out the issue I am currently having? I am extremely unhappy. I need someone to assist me ASAP!