MTN Over Charge

I had a contract with MTN for 3 years where I paid R389.39 every month up until this month when I got a huge surprise and was charged R2299. I immediately called in and asked about the Debit order and was advised they will look into it only to later get an email to say that my account is not in credit and therefore they will not refund. I logged it again today and the one lady I spoke to said it was and error and they will rectify it in the next 48 hours (Case number: BO***) They just basically rep**** that my contract has ended and I was charge normal rates. Now my concern is I understand that the contract have ended and the would maybe charge a little more then normal but not by so much especially after not informing me atleast. I had to upgrade my contract and when I spoke to the person that assisted me she told me that the price would stay the same and would not increase by much more should my contract expire and goes on a month to month. I hosestly don't mind paying but from R389. 39 to R2299 is way to much.