TERRIBLE CUSTOMER SERVICES!!!

I am really disgusted with the customer service I have been experiencing with this company, The company Norman Bissets has been collecting on behalf of Cell c , I last made payment last year (2021 September) and I completed paying , however cell c is still saying I am not paying , whereas I settled the outstanding balance with their collector. I did not recieve a paid up letter , this is very frustating because it is affecting my credit score and my name. I would like this retified , after many attempts to contact this company they have not gotten back to me , their lines are long and difficult to get through to.