O.K, I think I have a better understanding of what you want to accomplish. BTW, I have a smilar Panasonic IP Phone, the KX-TGP551, which is essentially the same phone.
IF you want to keep the Comcast Voice Line and be able to answer and make calls on the KX-TGP550 IP Phones (base Unit and Handset), what you can do is to get a OBi110 and connect the Comcast Line to the LINE port of the OBi110. You can now answer calls on the Phone attached to the Phone port of the OBi110.
Now, you will need to setup the OBi110 as describe below to to get incoming/outgoing to work on the KX-TGP550:
http://www.obitalk.com/forum/index.php?topic=1923.0and
http://www.obitalk.com/forum/index.php?topic=2454.0With this setup, you can back-feed the Phone Port of the OBi110 to one of your wall jacks so that other analog phones in your home will continue to work. With the setup above, you need a SIP provider [or a pseudo-SIP (127.0.0.1)] on SP2 for the above to work.
Of course, once you have the OBi110, you can setup GV voice on SP1 for free US and Canadian calling and you will not need the Comcast Voice.
You can avoid all this and just setup your IP Hone with a SIP provider and use it as a standalone System (no need for OBi110) or Comcast Voice. You could purchase some additional handsets for the TGP-550, so there is no need for any analog phones at all). You can even setup free GV calling on one line of the Panasonic TGP-550 using Simon Telephonics (I have 3 Free GV lines setup on my TGP-551, along with 3 other VOIP providers).