[Users] VoIP on virtual infrastructure and oVirt in particular

Hello, any feedback about VoIP softswitches on virtual servers? I found this positive whitepaper on VMware web site but it seems mostly related to Class5 softswitches (intended for work with end-users): http://www.vmware.com/files/pdf/techpaper/voip-perf-vsphere5.pdf I would like to know about real experiences and Class4 softswitches too (used for transit VoIP traffic between carriers). Thanks in advance, Gianluca