SSDA FIX MY CREDIT SCORE PLEASE

I had an account with MTN and I got handed over to SSDA. I arranged for monthly debit orders until i settled the account in June 2024. Now to my surprise in January 2025 I received a call that I am owing the company again i sent all the proof of payment that the account was paid in full last year June 2024. its March 2025 i still havent recieved any feedback from them and now the account is showing as arrears and affecting my credit record can someone assist me on what to do