Incorrect Billing

I am still not clear why I was billed R4300.00 on data while I had a 6Gig plus balance on my 24 month contract account. I did not even get notification that billing has started \because I had data\". But trying to reason this with the billing department has been fruitless as they all believe what their system gives them is what is correct. I am considering discontinuing my contract but I don't know what service provider to try now as I've always used MTN. I feel cheated because I was forced to pay to avoid credit listing.<br> <br> *impoverished*"