Unconfirmed charges/ contract migration against customers will/ without permission

After buying a data contract from Cell C, at the end of my contract I queried the cancellation process in December 2020 and put through my cancelling officially on 1 January. By 5 January 2021, I received confirmation that my cancellation request had been received and my contract would be terminated. The next month, 5 Feb 2021 I received a bill for more than double the amount of my contract. I tried to get hold of customer care and eventually found out that they hadn't cancelled but migrated me away from the special I was on to the full price of the data contract. I have tried numerous times to rectify this with them and get the same reply. "We have received you request. please note that the debit order will be end February as you have received benefits on the 1st. Regards Cell c Team". I have removed the sim and am not using the data. I am happy if they charge me pro rata up until 5 Feb even though my request was made to end it on the 1st. When I try to ask for a manager I am put on hold for over 15 minutes and eventually get transferred back to sales or get hung up on. This is *******. Why should I get a bad credit record for your mistake? Should you not get permission before migrating a customers contract and charging them double?