Bad customer service and unresolved refund for mistake made by cell c

I signed a contract with cell c for fibre in June 2020. The contract value is R599 incl vat. They billed me R729. Upon this they also charged me for an extra fibre contract witch is non existing. With other words paying for two fibre lines. I phoned them for the past few months to get this sorted and refund me or to put the money as a credit on my account with no success. I also send emails on which they reply that a consultant will phone me to assist, needless to say I am still waiting for someone to phone me. I am sick and tired because each time you phone in to customer care you must explain the whole story again and all that you get is we apologize for the inconvenience and will get this sorted and the excuses for this issue always differ from each consultant. I have been a customer of cell c for many years but I will not recommend them anymore.