pathetic CELL C service

I have taken out a contract with cell c two years ago which will be expiring in December 2022. When the contract was taken out io was given the option to pay the account by debit order or by eft. I chose the latter. Well needless to say ever since inception Cell C has been hounding me telling me that my account is in arrears as they have put on a debit order for which was not signed by me. I have sent them proof of payment on every occasion they called me and eventually I just refused to send them any further proof and that is after a whole year of doing this. Just two days ago I received another call from them stating that I am in arrears and these calls come frequently as that call me twice a day the whole week. I told them not to call me as this is bordering on harassment as I am not interested in entertaining any further calls from them. I have requested on numerous occasions to speak the supervisor on duty, but I get told that they are either busy or in a meeting and i then request that the person calls me but to date not a response which I refer to as really very bad customer service. I have told them that due to the pathetic customer service I have received, I will not refer and family member or friend to them and even not renew my contract with them.