3.9 - Linux, Mac, Windows
2.4 - Android, iOS
1.0.2 - Windows Phone
||Linux, FreeBSD, Windows, Mac OS, iPhone, Android, Windows Phone
||Voice over IP, instant messaging, videoconferencing
||GNU GPL version 2
Linphone (contraction of Linux phone) is a free voice over IP (VoIP) service and SIP client, currently developed by Belledonne Communications in France. Linphone was initially developed for Linux  but now supports many additional platforms including Microsoft Windows, Mac OS X, and mobile phones running Windows Phone, iOS or Android. It supports ZRTP for end-to-end encrypted voice and video communication.
Linphone is licensed under the GNU General Public License (GPL) and supports IPv6. Linphone can also be used behind network address translator (NAT), meaning it can run behind home routers. Linphone uses GTK+ for GUI and on Linux it can be also run as a console-mode application. It is compatible with telephony by using an Internet telephony service provider (ITSP).
Audio codec support: Speex (narrow band and wideband), G.711 (µ-law, A-law), GSM, Opus, and iLBC (through an optional plugin)
Video codec support: MPEG-4, Theora, VP8 and H.264 (with a plugin based on x264), with resolutions from QCIF(176×144) to SVGA(800×600) provided that network bandwidth and CPU power are sufficient.
Linphone hosts a free SIP service on its website.