Asterisk on OS X?
open source PBX, VoIP, IVR & voice mail VersionTracker: Asterisk - CVS 10-28-03
VersionTracker had the CVS release of Asterisk show up in the RSS feed that I read. I admit, I never really considered Asterisk on the Mac platform. Scott points out that Asterisk just posted their first 1.0 RC. I guess you could use an external interface rather one of the internal Digium products: they do have the IAXy.
Update: OK, this is definitely an OS X version of Asterisk, since some AppleScripts for managing it also got released. The company behind this is Sunrise Telephone Systems, based out of Japan. They have some impressive GUI config screenshots for OS X up ("Macsterisk"). Now I'm excited!