MTN is messing up my credit score

I have a contract with MTN and my payment are up to date, suddenly I see that I have an outstanding amount which I have no idea about,when I ask they don't the have answers from April till now they say I have to pay the amount.This is so frustrating because it is messing up with my credit score,I can't do anything because of MTN.They telling me about the billing credit which they didn't mention when signing the contract.