Looking for advice on local intercom/conference system

I have a slightly abnormal application compared to what you find when browsing around Asterisk/VOIP websites and I am looking for advice on choosing a distro or package for the PBX and advice on softphones.

This is for isolated partyline/conference systems with < 100 phones, usually more like 10 phones.
I have custom analog mics/speakers that will connect to some embedded board with some kind of VOIP client on it. The mics are all push-to-talk so they aren’t passing audio unless someone pushes the button.

I want the phones to automatically join the conference, so all people need to do to talk is put they ear to the speaker and all they need to do to talk is hold a button down. I’m hoping there’s an app in the debian or centos or whatever (ARM) repositories that can do this. Or at least an easy library.

Will getting up towards 100 phones on a conference cause problems? There will never be that many people talking or listening at once. Or how instantly can a phone connect/disconnect from a conference?

Should I use one of the fancy distros like Asterisk now, or just install some barebones asterisk. The server machine can be a real x86 machine.

For a simple conference, you’ll want something that supports ConfBridge rather than MeetMe, so that’ll be the later versions of Asterisk (1.8 and 1.10). When it comes to source versus GUI packaging, that’s entirely up to you as to how deep you want to get into the guts of Asterisk. If you’re just going to use the system for the conference, then there’s no reason for a full blown GUI to manage it. A decently sized machine (maybe 3Ghz) should be able to handle 100 people in a conference without a problem.

For the endpoints, that’ll be a little trickier and budget dependent. You can get an embedded flash ram based board with microphone in and speaker out for $100ish. I’ve not seen a specific application for what you want to do with it, but I know one can be developed if needed.

Thanks. That helps a lot.
Any pointers for client/phone software that I can configure to automatically start and call in? I don’t know where to start. For the moment I’m assuming we’ll use debian arm.
This looks interesting on first glance:

I didn’t look too deep at the app, but I don’t see why it couldn’t be modified to work.