No bill explanation

I keep try to contact Telkom about my bill and I'm not getting an explanation. A payment of R600 was attempted for a R400 contract. The chat agents disconnect the chat. I actually thought that I was getting somewhere with the one that I chatted to today until the chat was disconnected. I'm being told that there is a back(something) charge that is being app**** because of the payment that rejected. I accept the explanation but my last debit was successful and the advisor has told me that it is because my most recent debit rejected. So how do you apply a rejection fee on a bill that you are attempting? I'm seriously annoyed!