When I first got my Incredible, I set up unanswered call forward on my phone to my GVoice number by dialing these 3 codes, followed by my GVoice number.
Unanswered call forward
*71, *90, *92
When I switched over to the S3, I just downloaded Google Voice and set it up and the call forwarding was already set up for my number, so I didn't have to do that again.
So you would dial *71##########, press Send, wait for confirmation, press end and repeat.
*71 is for no answer / busy - conditional call forwarding
I can't remember what the *90 and *92 are for, but I found them on some web site 2 years ago (can't find it now) that talked about setting up google voice on verizon, and they work.
Anyway, that's what I use - people call my regular number and my voice mail goes to google voice. You should set google voice to "do not disturb" so once the call gets forwarded there it answers immediately and doesn't keep ringing for another 3-4 rings.
I do have to give out my GV number for texts though, since I don't have a text plan and want free texts.