VOIP conversion - phones in multiple rooms

You have to feed the house wiring out of the modem jack. If you have a jack close to the modem this should be easy to do, just make sure you disconnect the old feed at the NID. 

So I can just disconnect the incoming copper at the panel downstairs and connect the phone jack on the gateway to a nearby wall jack? That’s it? 

As long as it was a working jack yes. The white/blue pair should feed all the jacks so put your feed from the modem where the feed from the copper was. I can't see what you're talking about but all the feeds to the jacks have to be common.

Thanks. You mentioned the NID. I assume that’s the network interface on the side of the house. It might be easier to disconnect the copper at that point rather than at my patch panel in the basement.  It’s sort of a rats nest as the builder didn’t do a neat installation. 

Yes you can disconnect at the NID which can be on the side of the house. Make sure to insulate it so it doesn't make contact with anything.

Thanks. Do I need to login to the gateway and enable the phone service or anything, or just plug and play. The self install “kit” that they sent is very vague and pretty much useless. 

If you ordered phone service with the Internet all you should have to do is plug it in. Take a phone and plug it in and see if it works. If you wanted to keep your number make sure the number is correct.

We’ve had the fiber internet and gateway for a couple of years but just now adding the VOIP and migrating off the copper for the phone service. I’ll plug a phone into the gateway jack and see if it’s live as you suggested. 
Thanks for all the help, sad that AT&T doesn’t explain this in their documentation as I’d imagine it’s a fairly common question. 

Make sure you tell them you want to keep you number if you do. The service won't be there if you haven't ordered it.

Did what you suggested and it’s working great. Thanks for the help!