FreeSWITCH is flexible, and scalable and is a platform that can be configured as a soft Switch, PBX and or a Soft Phone. It can be run from FreeBSD, Mac, Linux and Windows. It out performs asterisk/trixbox easily in scalability, stability, and voice quality.
FreeSWITCH is a voice application developers dream come true.
I have several systems in production with FreeBSD, and from Windows Server 2003.