Linphone

Linphone (contraction of "Linux phone") is a free voice over IP softphone, SIP client and service. It may be used for audio and video direct calls and calls through any VoIP softswitch or IP-PBX. Linphone also provides the possibility to exchange instant messages. It has a simple multilanguage interface based on GTK+ for GUI and can also be run as a console-mode application on Linux.
The softphone is 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 GPL-3.0-or-later and supports IPv6. Linphone can also be used behind network address translator (NAT), meaning it can run behind home routers. It is compatible with telephony by using an Internet telephony service provider (ITSP).
Features.
Linphone hosts a free SIP service on its website.
The Linphone client provides access to following functionalities:
Open standards support.
Audio codecs.
Audio codec support: Speex (narrow band and wideband), G.711 (μ-law, A-law), GSM, Opus, and iLBC (through an optional plugin)
Video codecs.
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.