I have a friend that just got comcast service i know it's now the best service but its the best of what have in our area but we are having trouble finding a modem with a phone line connection as he wants to get rid of the crappy comcast gear the service comes with please post any info it all helps,
What are you trying to connect? I was under the impression that Comcast only used coaxial cable into the modem.
If you are asking about using an actual phone through the modem then I would either use their equipment or cancel the phone service Comcast offers and look for a VoIP service on your own such as Magic Jack or Vonage. You will likely come out cheaper that way as well.