It would not be possible for Obihai to implement this feature. When someone calls your VZ mobile number, the call is first processed by VZ; the features available are limited by what they offer.
You could set up "conditional forwarding" on VZ, sending unanswered calls to GV (instead of VZ voicemail). Such calls could be configured to ring your OBi, routing to GV voicemail if the OBi were also unanswered. However, it would then take ~50 seconds before a caller finally got voicemail; most are not that patient and you would likely miss many messages.
Alternatively, you could port your cell number to a GV account (new or current) and get a new number for the cell. Calls to your current number could ring cell and OBi simultaneously. However, there are several issues: You may face substantial charges from VZ (cancelling the old contract and starting a new one). Calls made directly from your cell would show the new caller ID, some contacts would return calls to that number and you'd be back in the same boat. You could avoid that by making all calls via GV (can be automated with an app), but you would experience lower quality, longer setup times and more latency.
If you ported your cell to Sprint, they have good integration with GV that I believe will do exactly what you want. However, IMO Sprint coverage, reliability, quality and data speeds are all inferior to Verizon.
Although far from perfect, if you can just convince your contacts to always call your GV number, it will ring in both places, using the setup you have now.
