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

e.164 International Telecominication Union standard for telefon numbers.
fxo Foreign eXchange Office (pstn)
fxs Foreign eXchange Station (phone)
tdm Time Division Multiplexing (circuit switched network)

1. Communication overview.

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