Inability to close off my account and send me a paid-up letter

I have been trying for days now to get Cell-c to send me a paid-up letter. I am applying for a home loan but it appears that Cell-c hasn't closed my contract account and currently showing there's still an outstanding balance. It's been a week now trying to get a proper department to send me proof that I no longer have an account with them. I have been phoning and been sent to all kinds of departments, only their legal department could confirm that there's not balance but they're unable to send me the paid-up letter because the account was never handed over to them. Now I'm stuck and every department keeps transferring me. My home loan application is about to be cancelled because I can't provide proof that I paid off my Cell-c account. This is really outrageous and I'm deeply annoyed.