Telkom bad service

I have a contract with telkom. I changed my banking details I called in to inform them earlier this month. The consultant couldn't help me, he said I must write an email. I did, didn't get any response for 3 weeks. I called again, the consultant told me the system won't be able to debit on my new account. I asked what must I do then? She said I was suppose to pay 10days before the debit date. The debit will go through the old account if it bounce back I will be charged a penalty. Why must I be charged because I will have money on my new bank account. It's not my fault that their system is slowly. I informed them on time. Why must the debit go through?