Closed contract still active

Hi, so my experience with Cell C was good the beginning of my contract, the issue started towards the end when I wanted to cancel it and move to prepaid, this was I supposed to April 2018. I cancelled my contract 30 days before the end date which is what they requested of me. I made my last payment and suddenly I'm in arrears and was handed over to their debt collectors. I kept sending emails and calling Cell C to explain to me as to why they say my account wasn't paid, to no avail. This carried on till December 2022. I then decided to pay off this 'debt' they say I have to the debt collectors January 2023 and was told that this account will be removed from my name in about 30 days from then. Today, 27 May 2023, I checked my credit score and this account is still active. I don't even know why because it's closed, and it's preventing me from getting credit elsewhere. I'm very disappointed because I really trusted Cell C to do a good job. Looking for a contract? Would definitely not recommend.