For our VOIP we use Asterisk ( www.sipcat.com ). Recently we migrated the server from a physical machine to a KVM virtual machine under Ubuntu 9.04
When searching around i found some problems around running Asterisk in a VM, but there were also some succes stories.
I kept getting the message:
rtc: lost some interrupts at 1024Hz
I was able to solve this by disabling APIC for this virtual machine, i kept ACPI enabled.