It is easy, the telephone output of COMCAST’s modem just works as same as landline, you should bring that telephone output from comcast modem to DoorBell Fon DP-28-C’s input From Telephone Company (either RJ-21 or screw terminals), then run the output of DP-28-C’s To Phones (either RJ21 or screw terminals) to all the phone jacks in the premise. The role of thumb the DP-28-C is working as a bridge of your COMCAST’s modem and all the telephone jacks. (see Diagram)