hi, I would presume you have Cable Broadband through Comcast. I'm not sure what the Telephony modem is, unless the device you're referring to has just power plug, ethernet plug, Coax Cable screw-connector and maybe a USB plug.
If that is the device you have, that's your cable modem, and it doesn't filter your network connection. Buy a cheap cabled or cable/wireless router, plug the router into the cable modem, then your computer into the router.
Am I on the right track?