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) (support dropped in 2014)
- 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
- Support for IPv6 - If the hardware, Android version, ISP and all other parts of the connections involved can handle IPv6, then Csipsimple can be used to make direct end-to-end ipv6-to-ipv6 calls.
As of 2011
, reviews are favourable.