No promises, but try something like this: SP4(18005551212)
One problem with trying to hit GV voicemail is that the initial call counts as "answered" the moment Obi accepts it for forwarding. However, if you're sending it out again via GV, that essentially second call will not count as answered if no one picks up and it will go to voicemail. GV gives you two sessions on one SP (or at least used to under XMPP), so if that's still true you can theoretically do this. OR, you could send it out on another SP if that one has GV, too. But, if you use one with a different carrier, no GV voicemail for you!
Good luck.