Well, for them to list it on the sidebar, on the Obi talk portal is extremely odd and well, frankly misleading. That aside. Here is what the link you posted reads:
ITSP x SignalingProtocol
Signaling protocols for this ITSP. Choose from:
SIP
Google Voice
This is under "Setting up the System" and then "Complete Initial System Setup with the Setup Wizard"...
Do you know why it lists Google Voice if it doesn't work with Google Voice?
On my set up wizard, as I pointed out above, I only get the SIP option not the Google Voice option that the manual suggests.