GNU SIP Witch

GNU SIP Witch is a free SIP server software with Peer-to-peer capabilities from the GNU Project.
It is the GNU implementation of the Session Initiation Protocol (SIP), which is being used for the routing of the calls.
Availability.
SIP Witch is released as free software under the terms of version 3 or later of the GNU General Public License (GPL). It is designed for Linux, Mac OS X, BSD and Windows and planned support for Android. In the popular GNU/Linux distributions "Ubuntu" and "Fedora" it may be installed directly from the standard package sources.
Technology.
SIP Witch is written in the programming language "C++" and uses the uCommon, eXosip and GNU oSIP libraries.
Features.
The software enables Voice over IP as part of a self-organising Peer-to-peer telephone network.
It supports features like call forwarding, call distribution, call hold, presence information and (text) messages, supports encrypted calls and also enables NAT traversal to establish the peer-to-peer connections.
History.
The SIP Witch is being developed since the 10th of August 2007 by David Sugar within the GNU Telephony project. The first version was 0.1.0. Version 1 was released on May 14, 2011. It is being used as a component of GNU Free Call, which is supposed to be an alternative to Skype.