Cell c bad service

Hellopeter,please assist on a bad service i received from cell c.i had a contract with this network and i did an early update.they requested certain amount before they can proceed.i was instructed to deposit into the account which i did,and they gave me the phone.two months later they started to debit more than our agreement,when i do follow ups they told me different stories every time i call.i finally went to the cell c store and one of the staff member told me that they didnt add the amount i paid into the account that's the reason they were debiting more,they added the money after my complain and it was already in arreas.i tried to make arrangements so that they can fix their mistakes as one of their staff accepted on the phone call that is their mistake but still iam responsible for my account.please assist i need to understand this(why im i suppose to pay for some ones mistakes as now they handed my account into debt collectors)