I suspect that your problem is an artifact of the way GV avoids a loop when it sends a call to a mobile and the mobile forwards back to it. I believe it's not related to the OBi -- you could confirm that by temporarily shutting down the OBi and making a unanswered call to your mobile from Gmail.
A straightforward workaround would be to call your mobile via another service, e.g. Callcentric or Anveo if you set that up for 911, but the call would not be free.
As a possible free solution, get a VoxOx account and set it up to forward directly to your mobile; you would dial the VoxOx number from home, probably via a speed dial. From the VoxOx app, make these settings in File -> Preferences, selecting Phone on the left:
General tab: Uncheck Personal Assistant, Call Screening, Present Incoming Call Options, and Dead End.
My Phones tab: add your mobile number; only Find Me and the box for your mobile should be checked.
Caller ID tab: click Show my VoxOx number.
First test by answering your mobile to confirm that all the screening stuff is really disabled, then try letting it go to GV voicemail. Note that this link will not pass DTMF.