Unable to make phone calls after contract upgrade

I upgraded my MTN contract, during the upgrade I was provided with a new sim card. From the time of upgrade, I am not able to receive phone calls to my number. When called voice mail activates before phone even rings. I also struggle to make phone calls, sometimes the call goes through, but the majority of the time it does not go through. I have logged multiple calls, different settings were app**** via the call center, but the problem persisted. Currently I manage with Whatsapp calls only. My data is not affected. MTN has not been able to resolve this issue for more than a year. I have even done a sim swop at my own cost based on MTN's recommendation and the problem persisted. I am not able to make calls which is the main purpose of having a cellphone contract and feel MTN is breaking their side of the contract of not being able to resolve this issue. I want to cancel this contract as they are unable to resolve this issue but keeps on being referred to the call center. Even if I use on of the MTN Stores they phone the call center and I have to explain the same problem again without any resolution.

I swopped the sim card with another pre-paid sim card and then I am able to make and receive phone calls, so the problem is not with the phone.

Would appreciate any help I can get, either get the issue resolved or get my contract cancelled without any penalties.