Well, it has been exactly one week since I reset everything per Steve's instructions, and things seem to be working flawlessly. The only problematic call I had all week was an outgoing call that resulted in a single beep instead of the call going through, but who knows if the problem was due to Google Voice or even on the recipient's end. Whatever the cause of that single problem, I had no further trouble when I immediately redialed the same number. And even during that one problematic call, I never heard the same error "The number your dialed has not received a response from the service provider", and the next call was successful even though I didn't reboot the OBi or my modem.
For these reasons, I consider the specific problem that prompted my original post solved. I want to thank SteveInWA, drgeoff, and everyone else who has tried to help me in my early days of trying out Obi devices. Even though setting up the device doesn't look difficult initially, many small things can go wrong, and the setbacks would be highly discouraging without the help of this great community. Without community support, I could easily see an inexperienced user running into an obscure problem and giving up.