Incorrect Billing and Lack of Service/Resolution

In November 2014 I requested and took out a 1000 minute contract with 1 GB data on the iPhone 6 with MTN through the Apple iStore at Pavilion. The aforementioned limits constituted the contract to which I agreed. The amount to be paid each month was agreed at R1400. I have since been paying R1400 each month. This month, May 2015, I was charged completely arbitrarily R8150 which was debited from my account. I reversed the absurd payment and immediately contacted MTN who informed me that I should take up my query with the iStore through which I obtained my contract. Upon confronting an iStore consultant I was told that the fault lay with MTN whose \system had failed"to implement the agreed upon limit on my data and that he would log the fault with MTN. I have thus been charged for data that I am completely unaware of using. This situation has not been resolved and I have been significantly inconvenienced by this occurrence."""