Cell C Sucks Again

I am being harassed by Cell C and their bulldog NDS Credit Management for an account that was paid up in May 2015. Not only is the account paid up but I overpaid them by over R500. Numerous emails requesting a statement has fallen on ineffective and *********** ears. Proof of payment has been sent to NDS Credit as well as Cell C but again the level of incompetence is very apparent. I am now receiving phone calls and sms messages from NDS Credit reminding me that the account is still outstanding and has been handed over. I am really sick and tired of repeating myself every time and nothing gets resolved. What is one supposed to do and how can this nonsense be stopped. Cell C seem to think that they are in a superior position and can threaten with legal action without even responding to my requests for statements and some sort of proof of what they claim I owe them. I am not even aware of the amount that I am being sued for. It has come to the point that when I hear the name of Cell C I want to throw up. I had a contract with Cell C for over 10 years and it has certainly ended in a most unpleasant experience. I have literally been forced to pay an amount that was under dispute.