MTN mischarges

I upgraded my contact earlier this year, it was supposed to be more data for less every month but my contract went up more than R40. I have emailed and complained on multiple occasions but MTN hasn't helped me at all. They still call me to try and upgraded my contract but they won't fix the problems with my existing contract. They continuously open up my query and not even an hour later it is closed again. I don't get any updates on what they have or haven't found either. I'm seriously considering changing my service provider . I think it is disgusting that a company can do something wrong and rather than rectify it they ignore you and over charge you.