First, the basics: what is free software?
Free software is a program whose source code (the instructions it is built from) anyone can view, use, modify and share. The Free Software Foundation sums it up in four freedoms: use the program for whatever you want, study how it works and adapt it, give away or sell copies, and distribute your own improved versions. Note: “free” refers to freedom, not price. It may or may not cost money, but nobody locks you in.
The term open source (as in open-source software) is almost a synonym with a different emphasis. The Open Source Initiative defines it with ten criteria: among them, that it can be freely redistributed, that the code is available and that derived works are allowed, without discriminating against people or fields. In practice, Asterisk, FreeSWITCH and Kamailio meet both definitions.
Asterisk: the phone system that fits on a server
Think of the switchboard at a building's front desk: it takes the call, greets you, asks who you want to speak with and transfers you to the right extension. That is a PBX (Private Branch Exchange, a private telephone switching system). Asterisk turns an ordinary computer into that phone system, with extensions, voicemail, call queues, call recording and voice menus or IVR (interactive voice response, the famous “press 1 for sales”).
Its story begins in 1999, when Mark Spencer, a computer engineering student at Auburn University (United States), started a Linux support company and wrote the core of Asterisk in a few months. He released it under the GPL license, the same one Linux uses (today, GPL version 2). His company was renamed Digium in 2001 and, in September 2018, Canada's Sangoma completed its acquisition; since then Sangoma has sponsored and led the project's development.
Asterisk releases long-term support versions (LTS, Long Term Support) every two years. According to the official documentation, as of October 2026 the fully supported branches are 20 and 22 (both LTS) and 23; version 24 LTS appears on the official schedule but, at the time of writing, was still listed as a pre-release.
FreeSWITCH: the engine for high volume
If Asterisk is a company's phone system, FreeSWITCH is more like the engine room of a large platform. It is a switching and media engine: it connects calls, mixes audio in conferences, bridges video and transcodes (converts audio between different formats) in real time. Because of its design, it is widely used when you need to handle a large number of simultaneous calls or build your own services on top of it.
It was created by Anthony Minessale together with a small team. The project was announced in January 2006 and its first official release came out in May 2008. It is distributed under the Mozilla Public License (MPL), which allows it to be used and modified in production at no cost. In 2017 that same team founded SignalWire, the company that maintains it today; the community version is on the 1.11 branch, published in SignalWire's official GitHub repository. One publicly documented use: the virtual classroom platform BigBlueButton states that it is built on, among other things, FreeSWITCH.
Kamailio: the air traffic controller for calls
To understand Kamailio, picture an air traffic controller. It doesn't carry passengers: it decides which plane lands on which runway and in what order. Kamailio is a SIP server (SIP is the Session Initiation Protocol, the language IP telephony equipment uses to start and end calls). It works as a proxy: it receives each call request, verifies who is making it and sends it to the right destination, without carrying the audio.
Its official site describes it as capable of handling thousands of call setups per second, which is why it is used as the front door for carriers and large platforms. Its origins are academic: in September 2001 the SER project (SIP Express Router) was born at the Fraunhofer FOKUS institute in Berlin, and released as free software in 2002. In 2005 Daniel-Constantin Mierla, Bogdan-Andrei Iancu and Elena-Ramona Modroiu created a fork called OpenSER, which in July 2008 changed its name to Kamailio due to trademark issues. It uses the GPL license version 2 or later, and its most recent stable release is 6.1.5, from October 15, 2026.
How they work together on a real platform
These three projects don't compete as much as it seems: they are often combined. A typical architecture puts Kamailio up front, receiving and distributing traffic, with Asterisk or FreeSWITCH behind it running the call logic.
For example: a customer of a chain of clinics dials the appointments line. Kamailio receives the call, checks that it comes from an authorized carrier and sends it to the least busy server. There, Asterisk plays the menu, checks the CRM (customer relationship management system) to see whether the number already belongs to a patient and routes the call to the scheduling queue or to an AI voice agent. All in a couple of seconds, without the patient noticing how many pieces were involved.
There are documented cases of this model: IvozProvider, a multi-tenant telephony platform developed by Spain's Irontec and released under GPL 3, uses Kamailio as the proxy and Asterisk as the application server. The engineering team at 19 80 TIC Cloud runs its own IP telephony platform on this same technology.
What your business gains with free software
- No per-user licenses: you don't pay for each extension or agent you add, so growing doesn't drive up software costs.
- Flexibility: it adapts to your process, not the other way around. Schedules, routes, menus and rules are configured to fit.
- Integration: it connects with your CRM, your ERP (enterprise resource planning system), WhatsApp or AI engines through open interfaces.
- Active community: thousands of developers review the code, report bugs and publish security fixes.
- Independence: you don't depend on a vendor keeping a product alive or forcing you to replace your equipment.
What it demands: it's not install and forget
Free software doesn't mean running it is free. A serious voice platform requires:
- An expert team: SIP, audio, networking and Linux systems are specialized disciplines, and a configuration error means silent or dropped calls.
- Ongoing security: telephony servers exposed to the internet face intrusion and toll fraud attempts (unauthorized international calls) all the time. You need firewalls, encryption and monitoring.
- Maintenance: each project publishes updates and security patches, and older versions stop receiving support on fixed dates. Someone has to plan and test those migrations.
- Infrastructure: redundant servers, backups, carrier connections and call quality monitoring.
Build it yourself or hire someone who already runs it in the cloud?
You can build your own phone system with Asterisk on an office server, and for a small pilot it works. But when telephony is part of the business (sales, customer service, collections), the math changes: you would have to hire and retain engineers, monitor security around the clock and be accountable for uptime.
A provider that runs this technology in the cloud has already solved that work: the platform, carrier connections, updates and monitoring. You keep the benefits of free software, with no per-user licenses and open integration, and pay for the service you use. To see how this works in practice, check out our cloud PBX, the call center platform or the AI voice agent options.
