Hello
I am curious if such a feature is possible. We currently have 1 SIP trunk and 1 DAHDI analog trunk connected to FreePBX with 8 SIP phones.
In the building, we also have 2 emergency phones that are connected to the DAHDI FXS.
Is there any way to configure FreePBX not to ring those two phones unless it detects a network failure? I remember one day our building switch went south and killed the network, turning off all the SIP phones.
I think we’re going to need a little more detail.
The answer is almost certainly “YES”, but I don’t see the use case that makes these phones do something…
Sure.
We are a volunteer fire company so we always need to be reachable. We have our primary phone system which is 8 VoIP phones connected to FreePBX which has 1 SIP Trunk and 1 Analog trunk.
We have 2 phones, one in the engine bay and one in the kitchen that are analog. I want to designate them the emergency phones that would only ring if the network were to die and take the Voip phones down.
I’m not sure of your e911 liability, but I know that mine is that 911 calls MUST get through or I am liable, I would look at a basic ATA like a sipura/linksysys/cisco 3000 series that can do that natively, if no network or no asterisk/FreePBX, it fails through and rings the FXS port to which you connect your loud ringer and big red phone on the wall ( I believe that the said 3000 will further failover to connecting the FXO to the FXS if it loses power , if you use something else you might also need a UPS also to power it for when your fire station catches fire and you lose power 
) . Your main phone line MUST be over copper/fiber FXO with your local telco , you can’t rely on network connectivity.
I should note though that I don’t know of any ATA that has UL and e911 approval , if anyone knows of one I would love to see that document.
You say that the two analog phone are connected to the dahdi fxs. Do you mean that they are connected on the FreePBX with an analog card? If yes then just make three ring groups, two with all the voip phones and the third one with only the analog ones. Set as destination of your inbound routes the first ring group with the voip phones, inside this ring group set as destination if no asnwer the second ring group with the voip phones and finally inside that one set as destination if no answer the ring group with the analog phones. In each ring group set the ringing time to maximum (60 sec).
Now if the voip phones are down the call will pass immediately the two first ring groups and ring the analog phones. If the voip phones are up then they will ring and having two ring groups will get you two minutes spare time to pick up the phone before the analog ones ring. Finally buy a ups for the freepbx and the switch, plus your internet modem.
Yes, they are connected via an analog card, no ATA devices.
I set it up like that today, and everything is protected with UPS (and our building has a generator).
Next steps will be to order a backup analog card and set up a backup server.