Bad Customer Service

I have a dispute on my contract and not happy with the service I'm receiving from Cell C.

My account was handed over to MBD for collection and I settled the account in July with the amount of R3920 and they told me my account will be settled and I'll get a paid up letter. I called the 2 weeks later I was advised the account is back at Cell C they are the one to issue a paid-up letters. I called Cell C on monday and the agent said she can see the account is paid up and she will send a request that they send me the paid-up letter, 3 days later I still have not received the paid up letter an i called them back, now the agents is telling me that there is a balance of R5000 and some rands still outstanding. And another thing that annoys me is that I have been receiving calls from Nimble demanding money where the account is settle through MBD.

I will not pay this as I was told that my account is paid up.