Check out this one:
Building an Asterisk Test Lab, Part 1
Asterisk and VOIP is way cool. This article provides clear instructions on how you can play with it.
"Deploying a new Asterisk PBX is not a trivial task, so the wise admin first sets up a test lab. This two-part sub-series will show you\
how to be up and running in an hour, and how to spend little or no money in the process. You should have knowledge of basic networking and\
Linux system administration."
VoIPowering your Office with Asterisk
http://www.voipplanet.com/solutions/article.php/3596101