ATT invoicing for interest payment on interest, even though bills are paid on time

ATT invoicing for interest payment on interest, even though payments have been made on time.

This billing problem has existed since an initial "fee" was charged without any notification several years ago.  Since that time, I have not been paying the interest on an initial undocumented fee, and each month, I am assessed interest on this unpaid interest - even when the actual alleged services rendered are paid on time.

The amount under discussion is purely and baldly company revenue for nothing rendered.

Please check your records for details.  Thank you.