Voip phone services

Asterisk phone

http://en.wikipedia.org/wiki/Analog_telephone_adapter

http://www.mikesoares.com/guides/asterisk/

Outside

http://forums.digium.com/viewtopic.php?p=60983&sid=8519cbb3869300c8c2b58639788f77d1

Certification

CCNA voice cerfication by Cisco, book