Billing error for over 6 Months

I have been given a settlement quote in May 2017 and the settlement amount was debited from my account. I have phoned to verify on numerous occasions to verify that it was a complete settlement on my account and now they are unable to provide me with a settlement letter. I have tried on numerous occasions and sent numerous emails. I don't even get responses anymore. It was a settlement on my African Bank Credit card and now I get told that it was only a partial amount and the account has been handed to a different collector so they are unable to assist me with anything on my account. This is still a negative impact on my credit score with each passing month and I cannot change a single thing because I simply just don't get a response. It has now been 6 months going for 7 and it has still not been corrected. Why are people employed if no one can do their jobs?????????