over billing

On the 24 of December 2014 I went to mtn for an upgrade the terms of my upgrade was R456 over a period of 24 months. After the first month my bill was 750 rands. I phoned in to customer care and they told me the consultant was supposed to ask me if he should set a limited, which the consultant did not. I then requested that the customer care agent set my limit, i was told everything was set, I would be billed as agreed. Next month I'm billed 820 rands! This time i go directly to the store. I'm then told again my limit was not set. I ask them to please set it and they assure me it's set. Month after that I'm billed 1020rands and it's been so for the past five months! !!!! Every time I phone or go in I'm told the same old story and then the consultants want to get more upset then what you are. I don't understand why I'm being billed so much. Once the data given to me is up I purchase my own airtime I don't use not 1bite more than what I'm given from mtn so why are you billing me so much? You guys are really messing up my debit orders. I cannot wait for this contract to end. I will never do business with mtn again!