BAD SERVICE OF MTN

I will never take out a contract with MTN ever again. I took out a contract October 2020 and it shows on my statements that I'm finishing off on the 16 October 2023. I phone them to cancel my contract they said, they cannot due to arrears, but I paid my last payment on the 13th of October. I ask them what arrears is that they say is for October 2022🤔 How is that possible