SIP

SIP: Session Initiation Protocol

http://www.ietf.org/rfc/rfc3261.txt

Blink http://icanblink.com/download.phtml Known to run on Windows, Mac, and Linux

Blink Client: Tested on Ubuntu 10.04, blink-0.2.11 SIP Server: Asterisk PBX v11.2.1, Ubuntu Server 12.04

Client Built from source.