I've just given 3CX a go, and I like what I see for a windows platform.
I managed to set up one extension and have it call through the PAP2T. Outgoing External and Incoming calls failed to work, but I can only assume that's because I'm not switched over to Open VFX.
Edit: I've also downloaded AsteriskNOW as an alternative. I'd prefer to keep away from running a VM on this particular machine.