Go into your Google Voice account and set your cell phones so they do not have direct access to your GV voicemail.
Google Voice web page > settings > Edit phone > Show Advanced Settings . Voicemail Access > No
Now when you call your GV number it will ring all your forwarding phones just like when someone else calls. If you want access to your GV Voicemail or the IVR press * when it starts to ring.
If you only want the call to forward to the Obi then you can put your cell number in your contact list and set it to forward only to Google Chat. With this setting you won't be able to call your GV number to reach any of the other forwarding phones, but you can still access your VM using the * when the ringing starts.