Flexisip

Flexisip is a modular SIP server suite written in C++17 that provides a full range of real‑time communication services. It includes a proxy for routing SIP messages, a push notification gateway that delivers incoming calls and text messages to mobile devices when the app is not foregrounded, a presence server for online status, a conference server for group voice/video calls, a B2BUA for identity translation and media transcoding, and a RegEvent server for registration notifications.

The software is aimed at operators and developers who need a self‑hosted, open‑source SIP platform for VoIP, instant messaging, and video conferencing. It can be deployed on standard servers for large‑scale services—such as the free linphone.org offering—or embedded on smaller hardware for device‑level communication. Compatibility with other SIP systems and support for high‑availability and load‑balancing make it suitable for both cloud‑based and on‑premises deployments.

Flexisip is released under the AGPL‑3.0 license, requires no subscription, and can be built with optional components like MySQL, Redis, SNMP, and OpenID Connect. Documentation covers supported RFCs, configuration, and packaging, and the project provides source code and stable releases for production use.