MTN Why?

MTN! A customer for almost 30 years and through MTN's bad administration and billing I have an account showing in arrears. How does this work MTN when my account is paid by debit order each month. After so many calls and frustration and consultants telling me it is all sorted I see a notice on my bank account saying Account in arrears showing MTN 1 month in arrears you may dispute with the credit bureau. MTN why???? I have always had a clear credit score. This is not acceptable.