Using comcast modem with existing phone sockets

Old 01-08-14, 12:03 PM
Thread Starter
Join Date: May 2013
Location: USA
Posts: 27
Received 0 Votes on 0 Posts
Using comcast modem with existing phone sockets


We used to be on AT&T for phone. We have the outside phone box, and then a network of internal phone lines/sockets for nearly every room.

When we moved over to Comcast, the phone line came out of the cable modem instead of from outside.

The comcast technician tried to plug the modem into one of the phone sockets upstairs and he said that should make every socket usable throughout the house, but this didn't happen. He said they wern't wired right because none of the other sockets had the signal.

In the end, I wound up just plugging the phone directly into the modem.

Was the technician right? Should plugging the cable modem into an upstairs socket make all sockets usable?

Old 01-08-14, 12:31 PM
ray2047's Avatar
Join Date: Mar 2006
Location: USA
Posts: 33,575
Received 14 Votes on 12 Posts
Was the technician right? Should plugging the cable modem into an upstairs socket make all sockets usable?
Only if all the jacks are wired to the same pair and wires of that pair are good. You need to check the wiring at each jack. First look at the color of the wires connected to the jack. They may be green and red or blue and blue with white stripe but other colors are possible. The key being all use the same pair color. Second thing is if there are two are more cables at a jack all wires of the correct color pairs must be connected together. Example if two cables using red and green both reds must be connected to one terminal of the jack and the two greens to the other terminals.

That the simple you hope for but it could be a mixture of cable types in which case you may see different colors at different jacks. Also the jacks have four terminals, usually R-G-B-Y. For your modem they must all use R and G. And both of the preceeding assume no home runs to the NID.

Not sure how clear and easy to follow I was so just start with looking for broken wires at the jacks. If you find no broken wires you can post back for further explanation.

Last edited by ray2047; 01-08-14 at 01:55 PM.
Old 01-08-14, 03:03 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
I need to ask the stupid question, when the Comcast installer tried plugging the modem into the telephone jack did he first disconnect the plug in the telephone company Network Interface Device? If yes, then did you get no dial tone or were you just unable to dial out?
Old 01-09-14, 01:41 PM
Join Date: Nov 2007
Posts: 391
Received 0 Votes on 0 Posts
I'll second the stupid question.. That was my first thought as well.

Even a landline phone circuit that has been "disconnected" by AT&T, may still be still required by law (depends on where you live) to remain electrically live and able to dial 911. So if the wiring is still connected from your inside jacks to the pole, the signals will clash and nothing will work. You have to find where the wiring from the pole meets the wiring from the house (normally in a gray box on the side of the house called the NID). Those are all different, so if you could take a picture of what you have (if you can see a screw/clip marked Customer Access, open the box and show us what's inside too) we can get you on the right track.

If all of the phone jacks were previously on the same 'line' (number) then there should be no other reason it wouldn't work - unless there is some issue with the actual jack itself that you are trying to plug the modem into or someone (ahemCOMCASTcough) monkeyed with the actual wiring.

Thread Tools
Search this Thread
Ask a Question
Question Title: