Incorrect billing and invalid overdue status on ac

I am truly disappointed and fed up with MTN! I have an contract with them for which I am supposed to be paying R509.00 a month. All of a sudden in Oct 2015 I was charged R632.99. When I called and enquired I was told that it was becuse it ws not properly credited and it will be fixed. It was fixed in Nov and I was charged R400 in Nov to cover the overcharging in Oct. But to my surprise it happened again in Dec and when I called I was again told the same story. I then decided to reverse the amount and pay the R509 as it should be, manually. Now, I am receiving smses and emails saying my account is overdue and the line will be suspended. I am tired of calling and getting reference numbers but the issue is never resolved! How can the account be overdue if it is debited every month by MTN??? If its the crediting problem, why am I being treatened with my line being suspended? Here are some of the ref numbers I have received: REQ*** and *** as given by your agents Moleboheng and Nosiphiwe. Today I spoke to Nohlanhla who seemed not to know what she was talking about which drove me to this platform. Should this not be fixed by Friday I will cancel this stupid contract!!!