The worst service provider

I had a contract with Cell c form 2018 to 2022. My account was paid in full and I asked for a paid up letter which cell c never sent. Two years down the line I get a call from a debt collector saying that my account was handed over, I then made a call to call c requesting for a paid up later again and since my account was on zero they promised to send it, which they didn't again, I then get bothered daily by this debt collectors , I called cell c again today only got them to tell me that they have handed over my account for non payment which is ridiculous. I paid up my account, why is this happening? This has to be the worst service provider I've been with.