Asterisk

Asterisk is an open source framework for building communications applications, described by its own project as middleware sitting between internet and telephony channels below, and internet and telephony applications above.

That framing explains its position in the market precisely. Asterisk is not really a finished phone system but the engine most finished phone systems are built on, which is why FreePBX, Issabel and a long list of commercial products all describe themselves as Asterisk based. Choosing Asterisk directly means building the system you want rather than configuring someone else's idea of it.

The project is emphatic about one thing above all others, and it deserves repeating: it is imperative to read and fully understand the security information document before attempting to configure and run an Asterisk server. That warning is not boilerplate. A misconfigured SIP server exposed to the internet is found within hours and used for toll fraud, and the bill arrives before anyone notices.

Development and testing are primarily on GNU/Linux, where it is supported on every major distribution, and it has been ported to and reportedly runs properly on other operating systems. The project maintains an official documentation site alongside the source repository. Asterisk is free and open source with no licence fee, funded through the commercial ecosystem built around it rather than through the software itself.