I had occasion to help set up a new Google Voice account and noticed that Google Talk was not available as a destination by default, so I did the usual trick of switching over to Gmail to place one call from the web interface. But because it was a new account, it started out with Hangouts enabled and not Google Voice, and no phone icon was shown! However there is a dropdown menu in the lower part of the left hand column, which in this case was slow to appear (Google was having some server issues), but once it showed up I could go into that menu and there was an option to revert back to using Google Talk. Once I did that, the page refreshed and the phone icon appeared, and then I could make the one call from the browser that sets up Google Talk as a destination in Google Voice.
This implies that if Google ever stops offering the option to revert to Google Talk rather than using Hangouts, it might not be possible to get a new Google Voice account and use it with an Obihai device, because it won't be possible to enable Google Talk as a destination. :'( Let's hope that doesn't happen anytime soon!
This is somewhat of a hot topic on the tech websites that follow Google and what's new their product offerings. One pretty good summary: http://www.theverge.com/2013/5/19/4346910/google-hangouts-upgrade-removes-host-google-voice-calls-gmail. The GV product Forum thread linked in the article is also insightful, in which a GV 'Community Manager' replies, in a soft of ambiguous way, IMO.
Your point that a newly created GV account defaults to Hangouts, rather than Google Talk, is pretty important. I would hope Obi updates their tutorials and videos so new users (or old users creating new GV accounts for friends and family) do not get tripped up.