Unable to cancel contract

I have canceled my contract with MTN in 2022. 2023 the account was active again and I was owing alot of money. I then contacted MTN to get this matter sorted out, after spending hours and hours on the phone, finally they assisted me and told me that it was their fault and I owe them nothing and assured me that the contract was cancelled. Ok fast forward this month , I get a notification that I still owe them. I don't know what kind of games they are playing because this is affecting my credit score,

Frustrated Customer