Haven't tried it on VirtualPC, but suspect it wouldn't work so well. I could see it working OK in a virtual environment on an Intel machine, being near-native speed, but for me VPC on PPC MacMini is so slow anyway... Guess i am better off buying an old PC laptop running TrixBox, like your friends... but frankly I don't really need a PBX, just interested to play around with one.
At the moment I'm using the asterisk-based free virtual pbx at
www.pbxes.org (and before that I used the somewhat similar
Voxalot - Home) and it's OK, but it'd be nice to be able to tweak things a bit more.
Maybe in a couple years this thread will be re-activated with discussion of the new Apple iPBX application in OS X Server with full iChat integration :-)
Ben