MTN skullduggery

For a number of years I was on a contract with MTN South Africa, on two numbers. One for a router, one a mobile phone. The time came when I no longer needed the contracts. I gave written notice within the 30 day period asked for by MTN of my intention not to renew the contracts but convert to pay as you go. I asked for, and received, confirmation that my contracts were cancelled, both by email and at the kiosk near to me. I asked for, but am yet to receive, a letter from the accounts department agreeing that the debit order on my account had been cancelled accordingly. The last day of my contract arrived, 11 December 2024, and I received notification I was now on pay as you go, and I duly put money into the account via internet banking. the next day, 12th December, both numbers were loaded with the data/time of the contracts. I immediately contacted them to say this was incorrect. Many, many, many communications later, I am still being charged an amount by them for a couple of weeks post-contract on, they say, one of the numbers, but the amount charged gives lie to this. They changed the end of contract date three times! How can that be legal? I am being threatened with the Credit bureau, suspension of services, they are still trying to draw money from my account - what do I do?