Settlement letter not issued.

I had cellphone contract with MTN which was handed over to debt collecters SSDA. Now the account is fully paid. Never missed a payment with them .Now they cant give the settlement letter or a statement that shows that I fully paid the account. They told me to contact MTN as the service provider still no help from their side with the settlement letter and is affecting my credit score.I am really not happy with the MTN customer service.