Trixbox

trixbox is a complete installer with CentOS,
Asterisk, FreePBX, mySQL, Apache,
and numerous other utilities, applications,
and script.

Basic features

Auto-Attendant
Ring Groups with mutiple ring strategies
Unlimited number of Conferences
Call Queues (waiting room w/ hold music)
Unlimited extensions & voicemail boxes
Call recording of incoming & outgoing calls
Customized hold music

                                                   and much more!