Cancelled contract

MTN - Follow up on my cancellation of contract with MTN. Received a statement reflecting September charges and surprise, surprise there are additional charges reflecting on my account, over and above my current contract. Called MTN three times today and no one was able to assist on these additional charges. I was told my cancellation of contract is not reflecting on their system, then I’m told I’m still been charged for the contract that was cancelled in July (end August been the final payment), yes I’m fully aware I’m billed in arrears and August should have been my last payment, third excuse was I did not pay July (which of course I did and my payment is reflecting on my statement. I gave 30 days notice beginning of July. On my third call I was informed that a team leader will call me to discuss, still waiting for that call and resolution. My statement clearly reflects device cancellation so why will I be charged end of September for a contract that is cancelled and paid up to date ie end August.