• audio with the following codecs: speex (narrow band and wideband), G711 (ulaw,alaw), GSM, and iLBC (through an optional plugin)
• video with codecs: H263-1998, MPEG4, theora and H264 (thanks to a plugin based on x264), with resolutions from QCIF(176×144) to SVGA(800×600) provided that network bandwidth and cpu power are sufficient.
• Supports any webcam with V4L or V4L2 driver under linux
• Any webcam on windows
• text instant messaging and presence (using the SIMPLE standart)
• Addressbook
• DTMF (telephone tones) support using SIP INFO or RFC2833
• understands SIP ENUMS (sip phone numbers using the naptr DNS service, without proxy)
• echo cancelation using the great speex echo canceler
• SIP proxy support: registrar, proxies, with digest authentication
• STUN support for traversal of UDP NATs (=firewall)
• sound backend using either ALSA (the most efficient), OSS, or arts (kde sound daemon)
• supports IPv6
• the bandwidth limitations are signaled using SDP (b=AS…), resulting in audio and video session established with bitrates that fits the user’s network capabilities.
• can use plugins: to add new codecs, or new core functionalities, such as remote directory search of sip addresses for example.

User interfaces

• Linphone is a nice GTK+/libglade application.
• Linphonec is a command line interface that can use readline on linux to get completion, history, such as bash.
• Linphonecsh is another command line tool to remotely control a linphonec daemon. Unlike linphonec, linphonecsh immediately exits once the command is executed.

Portability

• Linux/x86 and Linux/x86-64
• Windows XP and Vista
• Linux/ARM: without graphical interface. Linphonec or liblinphone are good candidates to provide the software stack of an hardware phone or hardware communication system.
• Linux/Blackfin: The uclinux.org project maintains a port of linphone for blackfin processors, without gui. Performance are impressive. See here for details.
• May work also on FreeBSD and OpenBSD with minor modifications to the build system.

Implemented standarts and protocols

• SIP (Session Initiation Protocol, rfc3261), the signaling protocol
• RTP (Realtime Transport Protocol, rfc3550), the real time streaming protocol
• RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals
• STUN (Simple Traversal of UDP NATs)
• RFC4629 (Usage of video H263 over RTP)
• RTP payload format for theora encoded video
• RFC 3016 — RTP Payload Format for MPEG-4 Audio/Visual Streams
• RFC3984 — RTP Payload Format for H.264 video
• RFC5574 — RTP Payload Format for the Speex Codec