It looks like PIAF seems to have come around on their avoidance of virtualization. When I last looked, they only supported Player, not ESXi.
PIAF is actually what I have, but being a PBX, it isn't completely suited for ITSP use. I could certainly see a place for it as a hosted PBX, though.
If anything, looking at PIAF again led me to rentpbx.com, where they have other systems mentioned.