Contact not canceled

I would just like to make an formal complaint against MTN for treating customers unfairly. My contract ended in Feb this year which i paid in full at the store. The lady that assisted me advice me that I need to phone 135 the following day to cancel my contract which i did. 2 weeks later i phoned again to confirm they advised me that my contract is closed. A few days ago i went on to the MTN app and saw an outstanding amount. I phoned today to query about the amount, seeing that the contract is paid up and canceled. A lady named Nikita assisted me and advise that she doesn't she any records of me phoning in no reference nothing now at this point you can just imagine how that made me looked. I then told her that i want to lay a complaint then she advised me i need to visit a MTN branch (pillar to post) which i will do very soon because i just felt that someone did not do what they were supposed to do now i have to pay for their mistake. MTN i please need this resolved because i do not owe you anything anymore. And please I bring this under your attention now, i dont want lawyers phoning me in a few months time saying i owe u. Thank you in advance Regards