Billed for a R500 data I did not use

Cell C has billed me for a R500 data I did not use now I am being harassed by the Cell C collections department. I had a contract with Cell C, on the 25th of October 2021, I bought the 6 months 50g data. On the 28th of October 2021, I contacted Cell C to get assistance with an upgrade since I was due for an upgrade in November 2021. This is when everything just went wrong. I was assisted by Nokulunga and she cancelled the data I did not even use. After receiving SMS' confirming the upgrade, I realised that something was not right because I was also upgraded to the wrong package. From that day I have been trying to resolve this problem but no one is willing to assist. When I send them emails, I receive phone calls from Cell C consultants who promise to investigate this matter and get back to me but I don't get any feedback except being harassed to pay the R500 of a data I did not use. All of the consultants I have spoken to are *********** and useless, they make empty promises. What puzzles me is that they could see the 50g data on their side and they confirmed that it was not used but on my side I could not see it on the app. At first they kept on saying there was a system error now they are just demanding that I pay without them investigating my problem. Because of the bad service, I requested the upgrade to be cancelled and I got confirmation that it was cancelled to my surprise it has not yet been cancelled.