Dear Cell C, I don't owe you anything!

I had an account with Cell C for years! Now I finally payed it off and closed it in Jan but I've been receiving about 3 messages a month telling me I owe them. I have tried to call almost everytime I receive the message to find out what I owe but I their call center agents keep telling me that I don't owe anything and that the message is a technical error. I'm tired of this and I've asked them to remove my numbers multiple times but nothing has happened. I got a call the other day from NDS saying it's regarding my cell c account. I told that lady I don't have an account anymore and I've closed it and she asked me who said asked me who asked me? Like? I hung up. So please remove me from all your data bases! I don't know who to call anymore.