You are making this much harder than it needs to be.
If you configured Google Voice on your OBi, then all you need to do is to call your google Voice number from the OBi, and it will go directly to the voicemail menu.
Are you in a country other than the USA? Are you diverting your inbound Google Voice calls to a Callcentric phone number?
The system should respond to touch tones. Did you check-mark the box to use the OBi with a security alarm? IF so, that will kill touch tone; don't do that.