Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Think back as to whether the error only happens for some numbers or one number, not by any means all. Faxing errors can be roughly broken up in to the ones in the machine at your end, the ones caused by the line (between machines) and the ones in the machine at the other end.
What is the exact error code, and what machine (by maker and model #) does it appear on?
Also, what type of phone line service do you have (e.g. landline/analog/POTS as opposed to Vonage/VoIP/MagicJack/UVerse/Comcast/digital) ?