MTN **** - Over charged for service not received

I upgraded my contract with MTN at Festival Mall in December where I asked for the charges to be capped at R500.00, initially it was about +-R494.00 but I was paying them a rounded off figure of R500.00 until July where the charges were R594 then in August it was R693 as well as September. I have not received any communication from MTN regarding the increases but only an SMS that I owe them and they will suspend my line, because I need my phone I paid them. I called them early September and I was promised they will investigate and that the charges will be capped at R500.00.I called MTN on the 22nd September William Meintjies who advised my bill was R693 for this month and when I asked why it jumped from R500 to that amount he advised that I have a service where I get 500mb data which is standard and an extra 300mb data, I then advised him that since I took out the contract I have been receiving 300mb and I have never received the 500mb which means I have been charged extra cash for nothing it seems MTN is busy ******** their clients then the line went dead.I want this sorted out ASAP and reimbursed as, they are quick to suspend my line for payments that are not due to them.