MTN debits incorrect amount

I upgraded my two top up account in January 2015. One for the Z3 compact for R499.00 p/m and a Z3 for R599.00 p/m. MTN keep debiting my account with incorrect funds. I should only pay R1098 p/m and i am debited over R2000. I want my account corrected and my money back. I will not pay for errors made by MTN. I always have minutes, sms's and data left over so how can I be paying more? This is ****************. The additional charges are on my *** number. First i receive terrible service from Amanda in the call centre now this. I called last month to sort this out but still nothing is done my ref number is ***.