The same thing is happening with me. I have GV configured so that incoming calls ring on my cell phone and my home cordless phones (through the Obi). About half of my incoming calls ring only on my cellphone and not my cordless phones. I am positive the callers are dialing my GV number, not my "hidden" cellphone number.
In addition, on outbound calls, I occasionally receive a 503 error. If I immediately redial, sometimes the calls go through.
This is a major problem that may be affecting everyone using the Obi with Google Voice.
Obitalk folks, please look into this!