Flexisip is a SIP proxy server implementation compliant to RFC 3261, written in C++11.

The project was started by Belledonne Communications in 2011. The focus was to develop a SIP proxy solution easy to install, configure and maintain, and offering “out of the box” all the required features to deploy a SIP service tuned for mobile applications.

FlexiSIP can be integrated into your SIP infrastructure for different purposes :

FLEXISIP in the cloud

Flexisip instances can be deployed on server machines, in order to run a SIP VoIP service. 
The free linphone.org SIP service is running with Flexisip since 2011, and enables Linphone users to create their SIP addresses in order to communicate together.

FlexiSIP embedded

Thanks to its modular architecture and its limited number of required dependencies, Flexisip can perfectly run on small hardwares (embedded systems).

FlexiSIP Push Gateway

Push Gateway is a specific operation mode of FlexiSIP, developed by Belledonne Communications to deliver SIP incoming calls or text messages on mobile device platforms where push notifications are required to receive information while the app is in foreground. 

Supporting push notifications was already highly recommended and becomes mandatory on some OS (Windows Phone, iOS10…). The push system is promoted as the efficient, highly reliable, bug-free system to receive information while the phone is off.

FlexiSIP Push Gateway aims at being integrated into existing SIP infrastructures that do not support push notifications, most of the time without any configuration change. 


Flexisip has the following main features:

  • transports: SIP/UDP, SIP/TCP and SIP/TLS
  • nat aware with built-in media relay module and stun server
  • digest authentication based on external SQL password database or static password file
  • registrar
  • routing based on registrar database or static route file, with forking
  • interconnected with push notifications systems for reliably notifying mobile apps of incoming calls or messages
  • high level event logging in SQL database for activity monitoring
  • high availability and cluster mode operation for large deployments
  • "store and forward" logic for SIP MESSAGE (IM) so that text message can be delivered when the recipient connects to the network
  • real-time statistics available through command line




Flexisip is distributed for free under the Affero GPLv3 license. The Affero GPL is an open-source license similar to the GNU GPL license, with new conditions adapted to server-side software, mainly that the source code of the software must be made available to users of the service.

Alternate proprietary licensing is possible with commercial agreement. Please contact Belledonne Communications for costs and other service information.

Binary packages

Packages for Debian and Redhat/CentOS are available.

Please read installation instructions on our wiki pages.

Source code

Source code is available from git.


(bc branch - includes submodules bctoolbox, sofia-sip, ortp)

git clone git://git.linphone.org/flexisip.git--recursive

Flexisip depends on sofia-sip stack, with minor patches applied, kept on git.linphone.org repository under "BC" branch.

oRTP is also a required dependency.



Documentation is available from our wiki

To learn more about our Push Gateway solution, see our documentation : FlexiSIP Push Gateway 

Mailing list

We invite anyone that wishes to get in touch with Flexisip's developer team to subscribe to flexisip-developers mailing list. Subscription is required to be authorized to post to this mailing list, in order to eliminate spam. Flexisip-developers mailing list is the place where to discuss technical issues and/or submit patches.