Google Hangouts calling operates independently of Google Voice -- there are no settings in Google Voice that control Hangouts calling behavior, and Hangouts has nothing to do with OBi operation. OBi devices use the old, "Chat" XMPP protocol, and they act as Chat clients. So, you must tell Google Voice to forward inbound calls to Chat, by checking the box to the left of Chat on the old, "Legacy" Google Voice user interface.
Note that the Google Voice initial signup flow has recently (within the past month) changed. New users, who do not yet have an inbound Google Voice number, will not be able to set it up on the old Legacy web UI. Instead, they will be intentionally directed to the new UI, which does not have a setting to forward to Chat. After obtaining one of Google's free inbound phone numbers, and then optionally porting in a mobile number, you will be able to go to the Legacy UI and check the Chat box.
Taoman, this is why I mentioned months ago, that I was reluctant to post the redirection link: while it does work today, only for established accounts, I don't know how much longer we'll be able to redirect via a URL. Google plans to shut down the Legacy UI after all of the essential settings and features migrate over. The billing section moved over recently, and the next thing to move over will likely be the voicemail settings.