MTN credit score

3 years ago I changed from a personal account to a business account, I late paid my account at the time due to the change over there was some confusion. I have the POP and yet according to my credit score I owe MTN this money. I have had debt collectors calling me non stop. I have sent MTN Legal many emails, they confirm I do not owe any money. Yes to date I still have my credit record to be updated and MTN Legal to reply to my many emails or update my credit record. I have tried to call MTN with little success. I have over 10 emails telling me they will get back to me within 24 hours. What a joke.