Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
What type of Comcast service do you have?
e.g. XFinity Voice
If it is XFinity Voice then you will need to check with Comcast if their service supports using the G3 machine for sending faxes - not all fax machines are supported by the XFinity Voice service.
Is the fax machine connected directly to a landline connection?
Have you checked with Comcast if their service supports using the G3 machine?
Did he check with head office?