Ekiga

Ekiga is a free Voice over IP, IP Telephony and Video-Conferencing application for Linux and other Unices (e.g BSD, OpenSolaris or MacOSX). It was written by Damien Sandras and is licensed under the GNU/GPL.

Ekiga is able to use modern Voice over IP protocols like SIP, and H.323. 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 audio and video codecs (including H.263+, H.264, MPEG4 and THEORA), and has wideband support for a superior audio quality, together with echo cancellation.

It also supports SIP/SIMPLE presence, and interoperates nicely with Asterisk.

• https://www.ekiga.org/: Web page
• Open Source VOIP Software | VOIP Phones