Hi
One of our customers has installed our reseller system for them, based on OpenVZ, and I know that in one of his virtualisations, he is running 210 concurrent calls. But, he has thrown a 16 processor, 32Gb of RAM, Raid 10 system at the problem.
It is usually better not to virtualise Asterisk, but not because it is Asterisk, but because it is VoIP, and when you share with other applications, you cannot control the bandwidth quite so easily, and some heavy bandwidth usage on another VM may cause degradation of VoIP quality on the softswitch, as may some heavy processor usage on another VM.
There are issues with timing sources, as Asterisk uses Dahdi as a timing source, which often cannot exist on a VM. we have worked round that issue, and Asterisk 1.6.2 releases another App for conferencing that does not require a timing source.
However, using Virtualisation can solve a number of problems, and may be a pragmatic solution, not withstanding the potential issues mentioned above. Another one of the products we have worked up which uses virtualisation is
http://www.star2billing.com/multi-tenant.
Yours
Joe