MTN charging after correct cancelation of contract

Called MTN to cancel my data contract and was told to call the next day as that was the correct date to cancel and avoid early cancelation fees. Called the next day and put in my 30 day notice to cancel. And was not told for any other fees. There should be no fees as it was the end of the term and I simply was not renewing. At the end of the month I get an invoice. When I query it I get no response. I get debited and reverse the debit and contact them again. Still nothing. A second month arrives and I get debited again. I contact them and reverse the debit. Now I get told that since the invoice was finalized before cancelation I am liable to pay. So the invoice was finalized before I canceled but if I canceled earlier I would have had to pay early cancelation fees. That is set up to make me pay no matter what I do. So following the correct procedure was pointless. I have asked for a copy of the recorded call and have been ignored in that regard