A while back OBI and Google Voice had a messy separation. Now they are happily remarried. http://blog.obihai.com/2014/09/google-voice-and-obihai-update.html
Setup takes about 5 minutes.
All you need is this $38 puppy: http://www.amazon.com/OBi100-Telephone-Adapter-Service-Bridge/dp/B004LO098O/ and an old school land-line phone to get this to work. (We use http://www.amazon.com/VTech-CS6419-2-Cordless-Silver-Handsets/dp/B004OA74B0/)
Going directly through Google Voice is better than paying a third party like Callcentric, which I had been doing.
Update - I guess you can't do E911 with two accounts linked up. Tiffany and I each use one of the two available slots, so none is available for E911 configuration. If this is a deal-breaker, then the Callcentric approach might still be best.