Billing and Accounts issue

Hi there, I just spend almost 30 minutes on the phone with your call centre and my issue is still not sorted. Let me first make it very clear that I do understand your billing procedure. July usage gets paid on 1 September 2017. Please don't explain it to me again. I've cancelled on of my Vodacom numbers on 6 June 2017. I was informed that the contract will end 31 July 2017 and the last debit order will be on 1 September 2017. Today I am told that the usage for August will also be billed and I will be liable for payment on 1 October. The gentleman could not resolve the issue so handed me over to his team leader, who kept saying hallo I cant hear you, and then she will try and assist and then she cant hear me. It is highly frustrating. I want to know why I should pay for August usage if I've got proof that the contract was supposed to be closed 31.07.2017, her answer: the system says so. Really?