Cellc for the love of God please fix you sh*t you guys did on my name at the credit bureau

It's been over 3 months since I got a call telling me that my contract with cell c is cancelled nd fully paid off. I asked for a settlement letter never got it. Today I checked my credit score only to find that I still have a account with cell c nd its overdue it drop my credit score. I paid the settlement amount cell c gave me last year September even I now I still have a account with Cell c can you please fix you the mess you guys did on the credit bureau on my name this has got out of hand now