Linphone is an internet phone or Voice Over IP phone (VoIP).

With Linphone you can communicate freely with people over the internet, with voice, video, and text instant messaging. Linphone makes use of the SIP protocol, an open standard for internet telephony. You can use Linphone with any SIP VoIP operator, including the free SIP audio/video service.

Main Features:

• SIP user agent compliant with RFC3261 
• Supports multiple calls simultaneously with call management features: hold on with music, resume, transfer... 
• Audio with the following codecs: opus, speex (narrow band and wideband), G711 (ulaw,alaw), GSM. Through additionals plugins, it also supports AMR, AMR-WB, SILK and iLBC. 
• Video with codecs: VP8, H264, MPEG4, H263, H263-1998, with resolutions up to 1080p on best machines.
• Video fullscreen mode
• Audio call conferences
• Supports IPv6 
• Supports any webcam
• Text instant messaging and presence (using the SIMPLE standard), with "is typing" feedback.
• Addressbook 
• DTMF (telephone tones) support using SIP INFO or RFC2833 
• Acoustic echo cancellation
• Multiple SIP account support: registrar, proxies, outbound proxies 
• Digest authentication 
• ICE - which allows direct (fastest) media connection when permitted by firewalls.
• Codec bitrate adaptation to available bandwidth for best quality of service