Contract closed but still being billed, no one at MTN wants to assist.

Called on 24 July 2023 to cancel my contract, which was already on month to month. I was told they need 30days notice which I was happy with. September 25 they took money from my account for a closed account. Since 25 September I have been calling weekly being thrown from pillar to post, customer service, accounts, retentions, no one wants to credit the September invoice, they all confirm they see on their system that I called on 24 July to cancel. Because I found no joy, I called my bank and reversed the payment in October. Now MTN is sending me threatening messages that I have been handed over to lawyers because of non payment. Why am I expected to pay a September invoice when I closed my account on 24 August? I’ve even sent emails and they choose not to respond. Pathetic, now they are going to blacklist my name for their mistake. I am furious!