You seem to be talking about a self purchased Telephony modem for phone and internet.
You need to first check to see if comcast allows you to own a telephony modem in your area. Then you have to make sure that the one you purchase is on the comcast list of approved devices. There seems to be only one telephone/modem without gate way that you can purchase if allowed in your area.
This model. I don't know as I never checked but one comcast care supervisor on the phone told me that comcast actually sells that model at some comcast stores as well.
http://www.amazon.com/gp/product/B00721TUNS/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
but verify before you buy with comcast.
To hook it up you would need to remove the other one. Hook up the cable to it and use a computer to connect to it directly by ethernet. Plug in modem and turn on computer. Let sit for a few minutes. Then open a browser on the computer. You should be redirected to an activation page. If not the address of the activation page is.
https://activate.comcast.com/RoutingEngine/welcome.do
If you can't do it online then you would call the activation department at 1-855-652-3446.
Comcast approved devices are located at this link
http://mydeviceinfo.comcast.net/