Cell c ***** and negative money impact on my credit report

I had a data contract with cell c, the debited double amount to what we agreed on.. then to get this in order it was a hustle.. I cancelled the contract and to get my money paid back was another hustle! 2 years later I’m trying to apply for something I found out that I can’t be approved because cell c has affected my credit badly and it shows that I owe them +R2000! I called the call center to ask for a paid up letter and I’m told it will take days before I get it, at this point I’m stranded because of the ***** they made with my contract!! This is annoying!! I hate cell c