Complaint - Bad Service

On the 26 July i made an early payment towards my contract for an early termination as i wanted to move to prepaid, Call centre gave me an amount to pay at the store, which i did, i then asked for a termination so i can be prepaid. On the 14th August i received my airtime as usual, i called and requested reversal, it took so many phone calls for this to be sorted and finally on the 20th August i was moved \prepaid". On the 25th August i received an sms saying i owe R218.97. How do i owe MTN that amount when i paid in full an amount given to me by them. I then called the call centre yet again"