MTN overcharging

In the last 3 month's my bill has suddenly shot up from its normal amount of about R600 to about R1200, despite not having used the data any more than usual. This month I received a bill of R2202.04. My credit limit with MTN is R1408, but still my account exceeds this?? What makes it even more questionable is the fact that because I have been billed more than usual I have used my data line even less to avoid this? My statement shows an amount of R1702 for \package switched data calls ". I have called MTN and they have lodged a query and have said that I need to call them back within 10 working days to find out what the outcome of the query is?? Really not impressed with MTN at all anymore. I feel their service has deteriorated and all they seem to do is increase their bills."""