Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
What maker and model of printer or fax-printer is showing this problem, please?
What phone line service do you have?
If it is 'landline' or POTS or ordinary phone line service, then I would advise you to call the phone company and ask them to do a line check for noise. It's a routine procedure, alerts them to possible issues with their service, they do it for free, and it could clear up the problem.
Ask the phone company to do a line check. It's the right thing to start with at least.
(If it doesn't solve the problem, let me know, at your convenience. And I like to know when my advice has helped.)