Thank you for reaching out through the forum. I’ll be happy to answer this for you.
Adding another line onto your Xfinity mobile account would not work for your fax machine, as that machine would not be a compatible device for cellular service. However, you could add a line to your residential services that could be set up specifically for a fax machine.
If you are interested in this option please contact us at 1-800-XFINITY and any representative would be more than happy to help set this up for you.
Thank you again for your question and if there is anything else I can do regarding your Xfinity Mobile service please let me know.