Refusal to cancel an expired contract

I had a 24 month contract with MTN which ended this May. Before the contract ended they increased the monthly installment by over 50%. When I called to make the number prepaid they told me that they will continue deducting the installment from my account until June. I do not know where that is coming from. I never agreed to that new contract. They told me I should have called to cancel which I did on 22 September 2021. And then again today 12 May 2022 a day before the contract ended according to my records. I am trying to get hold of the call centre and they keep cutting my calls. If anyone from MTN is reading this please stop the contract under my name and stop deducting money from my account.

Unhappy former customer