Mtn *****ing my money

Hi. I am very disappointed, been with mtn for years. I am due for an upgrade and I am actually considering moving to another service provider because of this. Mtn has been billing me for my data contract after I canceled it in December already , I phoned on thr 3rd of december to change the nr to prepaid. I have been phoning the end of every month, Mtn then promises to stop deducting the ekstra money and that they will pay me back but and that it will take 21 working day's.. Its almost 2022 and my issue hasstill not been resolved..... My new reference is .. *** all that happens each time. I phone is I get a new reference nr, other than that nothing!