Circumventing Google's voicemail is a topic of much discussion on this forum. If you search through the forum here (as well as on Google Voice's own forum), you will see lots of people asking for ways to prevent Google's voicemail from taking the call.
The closest we seem to have come is found in the following thread. Take note particularly of the various suggestions listed by QBZappy in this thread: Auto Attendant setup?
Hope it points you in the right direction,