debiting more than aggreed

I took out a contract last year June R729.00, i have not even had the contract for 12 months and they are now deducting R977.00 from my account, when i called to querie this they told me that i had taken up other contracts for R89.00 and another for an amount ofer R100.00 when i told the adviser that i did not take up any other contracts from MTN south Africa, i was told that i need to go and get an affidavid first before they can look into my query, how is this my issue to sort out, i did not sign for any other contracts, either you sort this out or i will stop paying my MTN account until you sort it out.