BILLING WRONG AMOUNT

I have a contract with MTN (Top-up 100 contract) since the beginning of the year (Feb/Mar 2015). First 2 or 3 months the billing amount was correct with an average of ?R250 per month but then suddenly in June 2015 an amount was billed for R1012.42. ?R800 more than it was supposed to be. After lots and lots of phone calls, emails to MTN and visits to the local MTN shop, the only explanation I got was that there was no Top-up for that month. I do not know how this is possible because it is a Top-up contract. I therefore only made the correct payment for that month which was R257.99 but because of this MTN has been disconnecting the service twice a month for being in arrears according to them. It is now almost 6 months later and I still cannot sort this out with MTN. Everybody at MTN seems to be just ignoring the complaint. I have to visit an MTN shop twice a month to reconnect the service. Absolutely bad service from MTN!!!!!