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

AoR Address-of-Record, a canonical address by which a user is known in SIP
e.164 International Telecominication Union standard for telefon numbers.
fxo Foreign eXchange Office (pstn)
fxs Foreign eXchange Station (phone)
ivr Interactive voice response 
moh Message On Hold
pstn public switched telephone network
proxy servers route requests to the user's current location
registrar registrations, User Agent Server handles registers
rtp Real-time Transport Protocol
sdp Session Description Protocol
tdm Time Division Multiplexing (circuit switched network)

1. Communication overview.

Asterisk
chan_zap.so
/dev/zap
zaptel
Hardware driver wctdm
hardware