Incorrect Billing and no Communication

I have two accounts with MTN The debit order every month is about R1170 for both contracts Now , I've had problems before. But this takes the cake. Last month my account was debited with only R470 (there were more than enough funds for the general amount of R1170. As I received my statement this week, it shows I owe MTN R690 for last month and the total that will be debited this month is R1890. Now furious , I phone the call centre. No one can tell me why they only debited R470 last month, eventually speaking to the debtors department this lady informs me, they decided to Bill me for next month as well. What??? Why??? And they can't reverse the debit order because it's been sent through already. MTN does not know what is going on with their accounts and different departments , there is no Communication either. It seems like everyone just do whatever they want. I refuse to pay MTN R1890 for their mistake and Ido not want to pay for next month in advance. I am more than happy to cancel both contracts and go to another service provider.