Recently my I had to reset all my network settings on my iPhone due to some dropped calls. This also deactivated Google Voice.
To reset Google Voice do this:
1) Login to Google Voice
2) Click “Deactivate Voicemail” under the phone line you want to fix.
3) Click Done
4) Click Active Voicemail and enter the number to dial.
Then it should work. w00t!
Reactivating Google Voice
May 3rd, 2010