2-line VoiP with new xFi Gateway modem

I've just installed a new xFi Gateway Voice/Data modem.  Internet works fine throughout the house, but the voice modem is not working with the same set up I had on my replaced Netgear modem.  I have a 2-line setup, with one of the lines going to a 2-line phone (a single 2-line phone cable).  The original Comcast technician years ago said it had to be routed using a Cat-5 cable.  When I plug the same set up into the back of the new modem Line 1 and Line 2, the phone system does not work.  Plugging an analog phone directly into each outlet works fine, so I know the modem is providing the correct signals.  Any advice?