Asterisk.org, I have been using it a lot recently and find it is
really nice PBX software. The config is a little weird at first, but
is really not that bad once you get used to it. It does full PBX
capability with voicemail, call routing, VoIP, etc, etc. Very well
supported by an active community.

It is Good Stuff. You will need to put linux or bsd on the old
machines. Depending on the vintage and available RAM, it could be easy
to install a newish distro on the old machines (just leave out the
GUI). Otherwise, Asterisk has only a few newish requirements that can
be built from source. IIRC bison was the one I ran into when doing it
on redhat 7.2. You should be able to install this on any pentium class
computer with 32-64m of ram and 500m-1g drive without any problems. I
run redhat 5.1 on an old laptop with 16m of ram, I bet it would run
Asterisk with a little work.

Check out
http://www.digitnetworks.com/store/
I bought 2 of their $30 OEM x100p cards. These are basically voice
modems, but do the trick. These are cheapo cards, but the only
drawback I can find is that they take one line per PCI slot -- not
much of a problem in this scenario. If you look them up on ebay, they
sell for even cheaper (2 for $40). They provide a CD with Asterisk and
some other stuff and install support for $5.

I use Asterisk with VoIP to make _really_ cheap long distance calls,
around $0.02 a minute. This might appeal to the homeless situation
where it is hard for them to keep in touch with family due to long
distance charges. Before I set up a softphone, I would dial into my
local Asterisk machine and dial from there, it then connects the call
through the VoIP carrier like a calling card.

Check out voip-info.org for a lot of really good info on VoIP as well
as generic Asterisk & config info.

I would recommend a 2 line system so that people checking their
voicemail would not block any incoming calls from employers. Also, if
they have access to computers, I have seen configurations that will
email voicemails as attachments when they arrive.

- Ben

PS I think this is an outstanding idea.
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