Flexisip is Belledonne Communications' open source SIP server suite for VoIP proxy routing, presence, push notifications and video conferencing infrastructure.
Flexisip is an open source SIP proxy and application server suite developed by Belledonne Communications, the French company behind the Linphone softphone. Written in C++ and compliant with RFC 3261, it routes SIP signaling for VoIP calls, instant messaging, presence and conferencing, and can be deployed by telecom operators, enterprises or hardware manufacturers who want to run their own private or public SIP-based communications network.
The project has powered the free linphone.org calling service since 2011 and was released as a standalone commercial product in 2015. It is dual licensed: the full source code is free under AGPLv3, while a proprietary commercial license is available for organizations that need to embed Flexisip into closed-source products.
Flexisip's modular architecture separates its capabilities into independent components: a Proxy Server for SIP message routing and registration, a Push Notification module compliant with RFC 8599 for waking mobile clients via APNs and Firebase, a Presence Server for real-time availability, a Back-to-Back User Agent for identity translation and transcoding, and a Conference Server for multi-party audio, video and group messaging.
Because the codebase has minimal dependencies and runs efficiently in C++, Flexisip can be deployed on both large clustered server farms and small embedded or IoT devices, a flexibility that distinguishes it from heavier open source telephony platforms.
Flexisip does not follow a typical SaaS subscription model. Anyone can download, self-host and modify the source code for free under the AGPLv3 license, as long as any modified network-facing service is also released under AGPLv3's copyleft terms.
Organizations that need to distribute Flexisip inside a closed-source product, or that want direct vendor support and access to enterprise modules like the Flexisip Account Manager, can purchase a proprietary commercial license from Belledonne Communications; pricing is negotiated individually and not publicly listed.
Yes. Flexisip's full source code is released under the AGPLv3 license and can be downloaded, self-hosted and modified at no cost. A separate proprietary commercial license is available for closed-source integration.
Flexisip is used as SIP proxy and application server infrastructure for VoIP calling, instant messaging, presence, push notifications and conferencing, typically by telecom operators, enterprises and hardware makers.
Flexisip is developed by Belledonne Communications, a French telecommunications software company founded in 2010 and based in Grenoble, also known for the Linphone softphone.
Yes. Flexisip implements RFC 8599 to deliver push notifications through Apple's APNs and Google's Firebase Cloud Messaging, ensuring reliable call and message delivery to backgrounded mobile apps.
Yes. Its modular, low-dependency C++ architecture allows Flexisip components to run on small embedded systems in addition to full server deployments.
Common alternatives include Kamailio, OpenSIPS, Asterisk and FreeSWITCH for self-hosted SIP infrastructure, or managed cloud communications platforms such as Twilio for teams that prefer not to self-host.