I'm not an expert at this by any means but you could give this a try: Go to Beta=>Simul Ring and set Ring All to your VOIPo and GV numbers. Then go to Voicemail and disable it.
I tried that before and it didn't work. I only got about 2 rings and it went to VM. I think it has to do with the voipo ringing the google voice which then tries to ring the voipo.
Bookmarks