Ok, I managed to get this to a workable state. In case others are trying to do the same, here's a short summary of what I did:
- Using the normal Obitalk setup, add the phone to your Obi account
- Add Google Voice (consumer) as the second provider
- Add the phone to your Dialpad account
This last step will configure your phone for Dialpad including personalizing the display. It also disables all other service providers and removes the phone from your Obitalk account. This seems to be done via an automatic provisioning setting. So even if you reenable Google Voice it will not work. To disable this, I logged into the phone's web UI and cleared the "ITSP Provisioning" field. After a reboot, I was then able to enable Google Voice in SP2. After another reboot, both services are now working.
Hopefully this is useful for others.