Nonsense!!!

I had a cellphone contract with MTN which expired on February 2015 due to the fact that i had financial troubles at the time i failed to make payment but then I later on settled my contract on March 2015 and I requested my number to changed to prepaid. Ever since i paid them off in March I have not received a paid up letter from them but this didn't't bother me as much as they were no longer debiting my account. All was fine until i tried applying financing for a vehicle and I was advised that I have been handed over by some company, I then went to Transunion and requested my credit report and to my surprise MTN has listed me on the itc stating that I owe them R138, this came as a shock to me because as far as I'm concerned my relationship with them ended in March and I don't understand where this R138 comes from as no one and i mean no one from the company tried to communicate with me regarding this amount. I have tried numerous times to get help from the call centre but I am getting no joy. I have thus decided to take this matter to the NCR as I feel that i have been unfairly treated by the company. MTN is just disgusting and unprofessional!! I am fuming with anger over this.