Billed more than agreed upon in contract

Hi on 2015/05/25 i registered a formal complaint with MTN,who responded with ref no MTN REQ *** that they will come back to me within 5 days.Till today i did not receive any correspondence from them after informing them via several branches that im billed incorrectly for the past almost 8 months.My original subscription contract was set at R329 per month but for the past months i paid any amount from R150 and more on this contract after it was set on only R329,they even set my hands to prevent me from spending more on data but the problem was never solved.I even request to cancel my contract.