(b)Cost sharing arrangement. A cost sharing arrangement is an arrangement by which controlled participants share the costs and risks of developing cost shared intangibles in proportion to their RAB shares. An arrangement is a CSA if and only if the requirements of paragraphs (b)(1) through (4) of this section are met.
Asterisk is a complete PBX in software. It runs on Linux, BSD, Windows (emulated) and OS X and provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in four protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.
Asterisk is known to run on many OS platforms. However, Linux is the main platform for development and Digium hardware support. If you are running VoIP only, or if you are comfortable with using external media gateways to connect conventional telephone equipment, then you have more systems to choose from, like FreeBSD, Mac OS X and Solaris.