Ekiga
Ekiga (formely known as GnomeMeeting) is an open source VoIP and video conferencing application for GNOME. Ekiga uses both the H.323 and SIP protocols.
Ekiga is compatible with any software, device or router supporting SIP or H.323. It includes SwissVoice, CISCO, SNOM, ... IP Phones, but also software like Windows Messenger, Netmeeting, SJPhone, Eyebeam, X-Lite, ... or also the Asterisk popular IPBX, as well as any other commercial or Open Source IPBX. Ekiga is not compatible with Skype.
It supports all major features defined by those protocols like call hold, call transfer, call forwarding, ... It also supports basic instant messaging, and has advanced support for NAT traversal. Ekiga supports the best free audio and video codecs, and has wideband support for a superior audio quality, together with echo cancellation.