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CSipSimple is a Voice over Internet Protocol (VoIP) application for Google Android operating system using the Session Initiation Protocol (SIP). It is open source and free software released under the GNU General Public License.
It relies on the PJSIP SIP stack and get features provided by this SIP stack.
The key features of this software are:
- Multi-codec support: Speex (narrow-band/wide-band), G.711 (u-law/a-law), GSM, iLBC, G.729 (support dropped with r2180, need to buy a licensed g729 pluggin), G.722, AMR (narrow-band), iSAC, SILK (narrow-band/wide-band/ultra wide-band)
- A plug-in adds support for Codec2, G.726, G.722.1 and Opus
- A plug-in adds video calling with VP8, H264 and H263-1998 codecs
- Multi-account support: up to 10 accounts can be activated at the same time
- Can use native audio driver
- NAT traversal using STUN, TURN and ICE
- Integration with Android operating system with filters and rewriting rules
- Security and encryption with SRTP, SIP over TLS 1.0 and ZRTP
- SIP-SIMPLE messaging
- An API for third party applications is available 
- Packet loss concealment (PLC) using PJSIP
As of 2011
, reviews are favourable.