This is something I have time to plan ahead, fortunately.
The county I volunteer with has moved from Centrex to VOIP. Right now they provided each station with an ATA and each station has a router and a Comcast cable internet connection.
There is a rumor that the County may provide SIP credentials to stations who have VOIP enabled phone systems once they complete the rollout.
The problem would be that the station phones and our SIP Trunk are on the internet we pay for, and this county VOIP would come from a whole different network.
Dare I ask if the solution is as simple as adding a second network card to the phone server?
You don’t need a second network card for this ( so far) based on your description. Is the PBX going to be behind NAT? How is thos second network coming in?
None of that requires you to have a second network card at this point. You can route things to the PBX over the network. You can put another network card in there but then you’ll have to setup route tables on the PBX to route your traffic out the right interface when requests or other packets are sourced from the PBX.