It seems there is a little hickup with the instructions google provides you with to "Activate Google voicemail no this phone". That is at least for the instructions for AT&T while trying to get Google Voice setup on an AT&T Galaxy S3. The instructions Google provides say to enter the following exactly as written as if dialing someone.
*004*1<your 10 digit phone number>#
Every time I tried the above instructions my phone would return an error code. I got so frustrated I even got online with AT&T tech support so that they could verify the state of things as I was trying various other commands that I found on the web. None, of the ones that worked allowed me to setup Google Voice as my voicemail provider appropriately. After being online with AT&T tech support for a while. I tried simply prepending another * character to the entire thing. Guess what, magically it worked. I even had AT&T tech support verify that it made the proper configuration change on my account. So the correct command that Google should be telling people to enter exactly as written is really the following:
**004*1<your 10 digit phone number>#
To disable voicemail forwarding I used the following MMI code.
Anyways, hope this blog post helps some other people struggling with this out there. I know when I was searching I couldn't find any solution though I did see a number of message boards with people looking for an answer to this problem. If I were a better person I would probably go back and update all those message boards with my solution. I just can't bring myself to do it.