Account in arrears even though it was settled and closed

I've just received a call from Cell C collections department informing me that my account is in arrears. Of course, I was very surprised because I paid the settlement amount and closed this account last year already. And their response to ICASA when I logged a complaint with them about the very same account was that the account was paid in full and closed.

So which arrears are they referring to?

Why is it so hard to close off a fully paid account? How many games, going back & forth, must one do before they close the account? I WILL NEVER TAKE ANY CONTRACT WITH YOU AGAIN, I just want this account closed!

Do they even know how much these incorrect entries affect one's credit score?