VOIP works with no Internet Calling on Today Screen
I have an HTC P4000 (Titan, PPC6800) on the Telus network. And by installing WM6VoIP.CAB and SetupVoIP_v0.3.CAB, I was able to go into the SetupVoIP app and on the last tap, set it to "Whenver Available." This causes VOIP to become active if there is a WiFi connection. Nothing shows up on the Today Screen, but in the Phone screen, TELUS changes to VOIP (or whatever you name the SIP connection). After that, all calls are being routed through the voip provider.
The volume of the rear speaker (and earpiece when connected) is rather low, but definitely audible. That's the last piece of the puzzle I have to figure out.