I agree with Ed. In my opinion this would not be the proper application of VoIP. Can it be done? Sure. But I think the practical limitations of ethernet make it un-economical.

Not to mention I intentionally stay away from lifeline services just because of the potential liability.

If I were going to approach this job, I would do some creative CID naming conventions and use channel banks. That way looking at the CID automatically tells security where the call box is located at without having to use map's. Not to mention it would be so much easier to manage a multitude of single-pair cat-3 then cat-5.

Sometimes it's just best to not re-invent the wheel. I cant help but think the VoIP units were designed for companies that already had some large vestment in a VoIP PBX and not really for a stand-alone system.