I bought majic jack for fax purposes only... 40 bucks for 2 years... works like a dream...its a usb adapter on one side and RJ11 (standard phone jack) on the other side... completely configures itself... you have a choice of phone numbers.. works with any phone.. (ive used different models.. and different faxes)... and outstanding quality.
PS.. I have run with both office jet 6110 and currently with office jet pro 8500.. standard phone cord... no issues with either.
You need to be aware that some of the HP printers with fax capability require a special phone cable (usually supplied with the printer) or an HP proprietary adapter. HP says that the phone cable is a single line cable where most of the ones you buy off the shelf are wired for two lines. Why this would matter, I do not know, but it will surely cause you grief if you do not use the correct phone cable or the adapter.
If you have Comcast telephone service, you should have an EMTA or Telephony modem with two telephone jacks on the back L1 and L2 (These are 6 pin RJ11 Jacks) and an RJ45 jack for your internet connection.
In this case, you can indeed split the L1 jack and hook up a phone and a fax.
A better solution is to get a second line for the fax and run the fax off the L2 jack
If all you have is an RJ45 jack on the back of your modem then NO you can not split that. (RJ45 is an 8Pin Jack)
In this latter case you can hook a Vonnage Modem'router onto the RJ45 jack and then whether Vonnage can handle a Fax is between you and vonnage.
So that brings us down to "What kind of Service do you have?", and "What make and model of Modem and Router do you have?"
Do you also have phone service via Comcast? Or use VoIP from another provider?
Since the fax capability won't work over a straight cable connection (obviously), you should just need to set the printer up as though the fax connection is just another phone line. (I realize that's what you're asking how to do, but you mentioned the "RCA Broadband box"--which is what prompted my preceding questions.)