OpenSIPS, short for Open SIP Server, is an open source SIP server that carriers and internet telephony providers place at the heart of their networks. It was born on 4 August 2008, when Bogdan-Andrei Iancu announced the project together with a first release numbered 1.4.0, presenting it as the continuation of OpenSER, which itself grew out of the SIP Express Router created in Berlin by the Fraunhofer FOKUS institute.
Development is led by the team at OpenSIPS Solutions, a company that, by its own account, employs the founder and the main developers of the project. The code is released under the GPL version 2, with an exception that allows linking with the OpenSSL library. The 4.0 branch, declared stable on 23 June 2026, is now at version 4.0.2, released on 8 September 2026.
What OpenSIPS is for
OpenSIPS deals with call signaling: registering phones, setting up and tearing down calls, presence and instant messaging. Its website insists that it is much more than a plain proxy, and that is the core of its identity. Besides acting as registrar, proxy, redirect server and load balancer, it includes a back-to-back user agent, or B2BUA, which lets it hold both ends of a call in order to change it, transfer it or bill it. It can also bridge SIP with SMS or XMPP. Carrying the voice itself is left to an external media relay.
Configuration is written in a scripting language with a C-like syntax that describes how each request is handled. More than 120 modules, according to the project’s website, add database access, dynamic routing, dial plans, gateway management and clustering. Management, event and statistics interfaces let operators control the server without restarting it, and the project also develops OpenSIPS Control Panel, a web administration interface whose version 9.4.0 shipped in August 2026.
Its kinship with Kamailio is a frequent source of confusion. OpenSER was founded in 2005 by three developers, Bogdan-Andrei Iancu among them. When the project had to change its name in July 2008, part of the team carried on as Kamailio, and a few days later Bogdan-Andrei Iancu launched OpenSIPS, citing irregular releases and weak quality control at OpenSER. Kamailio went on to merge the SER code back in and opened up to languages such as Lua and Python. OpenSIPS took its own path, with tighter governance around a core team and a stronger focus on application features such as the B2BUA. The principles remain similar, yet scripts and many modules are no longer compatible.
The release rhythm is another distinguishing feature. The development page plans one major release per year. LTS releases, usually even-numbered, are supported for three years and the others for eighteen months. Today the 3.6 series plays the LTS role, at version 3.6.9. That predictability suits operators who schedule upgrades far ahead for SIP trunking, carrier interconnection or session border control.
Who is it for?
OpenSIPS targets carriers, telephony service providers, contact centers and integrators building large VoIP offerings. Teams that want a single component for routing, load balancing and a few application features will find it a good fit, especially as commercial support is available from the people who write the code.
The barrier to entry is high. You need to master SIP and the logic of routing scripts, and OpenSIPS offers no voicemail, audio conferencing or interactive voice response. Control Panel eases day-to-day operations but does not replace writing the configuration by hand. For a company phone system, a media server like FreeSWITCH is a better match, with OpenSIPS added in front later if needed.
Download OpenSIPS safely
Everything goes through opensips.org. Its Downloads page gathers the source tarballs, the official apt.opensips.org repository for Debian and Ubuntu, yum.opensips.org for Red Hat, CentOS and Fedora, the opensips/opensips Docker image and the project’s GitHub repository, where you can clone the 4.0 branch, 3.6 or the development branch. These packages target Linux, and there is no Windows or macOS version.
The software is completely free, with no paid edition. What OpenSIPS Solutions sells are services such as support, custom development and the eBootcamp scheduled from late November to early December 2026. A branch that has reached the end of its maintenance period gets no further fixes or packages, so tutorials pointing to old third-party repositories should be treated with caution.
Frequently asked questions
Which OpenSIPS version should I choose?
For a deployment meant to last, the 3.6 LTS branch, supported for three years, is the safest choice. The newer 4.0 brings the latest work on TLS, TCP and profiling. The website always lists which branches are still maintained.
Is OpenSIPS free?
Yes. The server is released under the GPL version 2 and no feature is locked behind a paid offer. OpenSIPS Solutions sells training, support and custom development, all of which are optional.
Can I move from Kamailio to OpenSIPS?
Yes, but the configuration has to be rewritten. The concepts are close because both servers come from OpenSER, yet the syntax, some function names and the module layout have evolved separately since 2008.
