Thank you for your inquiry. I have listed the fax machine connections for Comcast phone service. Please be aware that Comcast doesn't support fax machines but rather the dial tone of the line.
The fax machine has to be 14,400 Baud Rate (or 14.4 Kbps) only The customer must use a 2-Prong Phone Line only The fax machine must be directly connected to the Line 1 port of the modem, and then the wall jack (or phone) is connected to the fax machine.
On my HP 6500 wireless all in one I am able to get direct faxes on a separate line by asking Comcast to
disconnect call waiting, forwarding and voice messaging for the separate fax line. This info was noted in my HP instructions and once Comcast did this all faxes are able to be sent and received as with ATT&T or anyother phone company provider.