MTN Billing Me For Account That Should Be Closed

I contacted MTN in October, 30 days prior to my contract ending, to cancel it because I didn't want the upgrade. After calling, i was told that the closure would be actioned. When I received the bill at the end of November, i called them again to verify, and I was told that the Cancellations department would action the closure, but i needed to pay the outstanding amount for them to close it. I paid it in immediately. In January I received the statement for December billing me, and I contacted MTN to find out what's going on. I was told that the account still had not been closed