Junk administration by Mtn

I canceled my contract with Mtn on 28 July 2020 and paid the cancelation fee and even got confirmation that it would be canceled. On 1 January 2021 I learned that my number is suspended due to my account being in arrears. I contacted the contract retention department and it was confirmed that I did follow the right process as a client but the transaction was not completed by their employee. I was transferred to the credit department to open my line but they couldn't unless I made an arrangement to settle the bill which was not my fault... My line is still not working and I have missed so many calls, I don't even know how much more have I lost from this incompetence...