MTN declines contract termination

I have been using MTN for years, pity I no longer own their contract sim-card. I tried several times phoning to plead with them to terminate, this contract is affecting credit ratings, Im tired when you use whatsapp number there’s no option for contract termination, when the number goes through systen is off for termination. When you dont pay, they call you every now and then. I no longer need their service