Telkom ******* charges

I have a cell phone contract with Telkom. Three months in the contract I changed banks but did not close the old account. I immediately notified Telkom and other debitors about this. Surprisingly only Telokm didn't update my banking details. Upon realising that Telkom was still debiting from the old account, I phoned them a couple of times requesting for them to change the banking details.

Last month it happened that there were no funds in the old account. Telkom sent me an invoice which I paid via EFT. In this month invoice I was surprise to find that I was charged for debit rejection. My question is why do I have to pay this charge if it is Telkom's fault. they failed to update my banking details even after notifying them several times. I have phoned Telkom more than three times requesting