Go into settings, and select the "Phones" tab.
Select "Add another phone"
Enter your new provider's number.
Select "Show Advanced Settings"
Under "Direct access to voicemail when calling your Google number from this phone?" select "Yes" and PIN not required.
Adjust other settings to taste.
Save the settings.
Now if the ObiHai can be configured to use a custom dialing prefix, you should be able to dial out from your GV number doing something similar to how we used to have to customise modem dialing strings:
So to call 816-555-1212 you'd need the number to be dialed as follows: <your gv number>,2,8165551212
where the , represents a short 1-2 second pause.
Just wondering if this is possible. I don't know anything at all about VOIP, so I could be way off base.