Overcharged for the second time by TELKOM

I am Tshepiso Matjila, this is my second complain about Telkom overcharging me, the first time they said they will look into it but they never got back to me, i still say our agreement over telephone conversation with Mondo Telkom was i will only pay R179.00 per month, September month they debited R512.48, today i got a statement that say they will deduct R484.47, why are they doing as they please, dont i have any say or rights to the contract that we agreed on as a customer? i can only afford R179.00, i dont understand why am i over charged because i do not finish the data they give me every month. If R179.00 is not the amount i will pay every month i will rather cancel the contract with Telkom and do business with other networks.