FreePBX is an Opensource Phone system that is customizable, at the core "at least with the official ISO" it is CentOS 7 and enterprise version of Linux and the core of FreePBX is the original standard for PBX servers known as "Asterisk".
That said, it is about as solid as it gets, if you choose to experiment with the FreePBX environment you could install it on your pre-existing CentOS desktop or server or take the best approach would be to use the official distro found here: download FreePBX
There have been many improvements to security in the latest edition, try to use the newest version for best practice and to avoid obsolete modules and protocols.
We offer professional installation of FreePBX for business that need stable phone systems and simplified communication. Find out more