mtn takes your money without permission

I have been with MTN for years with no problems, but last year September, I upgraded one of my accounts, was plan and simple R299 pm data contract, for which I signed, but every month i saw they deducted R799 from my account, i went and asked the branch where we did the upgrade, the man who helped me, put a request in to fix the problem, nothing, i have phoned many times and every time they ask for the front page of the contract , which I send to them, and still nothing, now this month for the second time, they deduct R3890 from my bank account, telling me I am in arrears, so how i fix it, if they don't want to help me and how can a business deduct more than what you signed for?? so now for a full year i am waiting patiently but now it is getting really out of control,