If you want to test an Asterisk based PBX, then trixbox is the system to get. trixbox now owned by Fonality, is a full fledged SIP based VoIP phone system. You can download a pre-configured VMWare image of trixbox 1.1 here http://www.vmware.com/appliances/directory/49.

trixbox Community Edition (CE) began in 2004 as the massively popular Open Source IP-PBX project named Asterisk@Home. Since then, it has grown into the world’s most popular distribution of Asterisk with over 65,000 downloads per month. trixbox CE is known for its flexibility to satisfy the needs of custom deployments and will continue to be FREE (as in beer and freedom).