Bad Customer Care Service at Cell C

I have phoned the customer care service of Cell C to change my banking details. The consultant told me that they cannot change my banking details, because my new bank account is less than 3 months old. I have explained to them that my salary is being paid into the new account and no longer in the old account. They said that I should pay the extra costs should their debit order not going through. I have sent them my electronic payment of the account. I have phoned their customer service again and explained to them that I have made the payment and that they should stop the debit order on the old account as I am not receiving my salary in the old account, but the new bank account. The consultant says that I should pay the extra costs should the debit order not going through. If the payment that I have sent the proof through, does not show in their bank account they cannot stop the debit order. This is really bad. All my other creditors did change my banking details to the new one and put through their debit orders on my new account. What is wrong with Cell C for not doing the same?