Mtn account in arrears while the account is paid up and closed

I'm harrassed by MTN, i had a contract which is paid up and closed. But there didn't update my payments credit bureaus, it appears as in arrears. The account was closed 5 years ago but it still appears as owing them. When i call call center they cant help me i must go to branch. When i get to branch I must call a call center. I failed security questions simple because I opened the account 8years ago, life has revolved i don't remember if i was paying via debit order or EFT. Disaster now my credit score is affected because mtn account is in arrears.