Just an update on the Asterisk PBX project.  I now have a working PBX, this is only configured for internal calls (at the moment we only need this for conference calls).  The installation is pretty fast using the Asterisk@Home install CD, CentOS is the underlying distribution installed (this is basically RHEL with the RedHat bits removed) which allows easy updates via YUM.  I initially had some issues, although the system seemed to be functioning properly, no sound output was generated (music on hold, menu speech etc).  After a lot of hunting around and a lot of trial and error, this was tracked down to the installed Zaptel PCI card.  We installed the card with a view to later look in to PSTN intergration, however it seems Asterisk was a little unhappy about having the card installed but not used–Removal of the card solved all issues!  The call quality between two softphones is pretty good; today I will test the conferencing facility with 6+ callers.