Asterisk

Mark Spencer at Digium.

Download source.
http://www.asterisk.org
Mailing list. Commercial , development, user.
http://list.digium.com
Wiki.
http://www.voip-info.org
Documentation project.
http://www.asteriskdocs.org

fxo Foreign eXchange Office (pstn)
fxs Foreign eXchange Station (phone)
tdm Time Division Multiplexing (circuit switched network)