Mtn can't give answers, still getting invoices with incorrect amount due

Received the same wrong invoice for the second time, Alice Van Der Merwe who happens to be dealing with my query is not even responding to my emails. I'm supposed to be paying R250 every month however R900 was debited from my account in April and now it's may I have to pay the same amount. To be exact the R900 that was debited amounts to 3 months payments. I just don't know what to do anymore. Do I have to take legal actions or what. All I have been told is that my contract was incorrectly captured and it will be rectified according to the package I signed for. What about the excess money they already debited from my account? When will I receive a correct invoice. They say the matter is being investigated but the bill is still running at a wrong amount. Mtn can't even give explanation except to say "a query had been Lodged" I need answers.