UNFAIR BILLING

I app**** for a contract earlier in the year using my previous banking details and it was declined. I than did another application in June and had new banking details. I went to the branch with my new banking details and also filled an application form which I saw the consultant scan and the application was approved. For my first billing I received an SMS stating that my account was overdue, I called the contact centre and the agent advised that the banking details that captured was for my previous account, I than did a manual payment and called again and asked an agent to change the banking details, I was told it was changed. For my second billing I received another SMS saying my account is overdue again. My last payment I was debited an additional R400 and the reason was for penalties . I called again and a case was long I was told it will take a maximum of 72 working hours(9 days) from the 15th September . I don't understand why I am billed for something that is not my fault, I have to pay for the agents mistakes.