In order to make Google Voice work correctly with the iPhone, three rules need be established about how the phone reacts to sending voicemails under different conditions.  The three rules are:
1.  No Answer
2.  Call Declined/Ignored
3.  Phone Off/No Service

If you fail to set up any of these three rules, the iPhone defaults back to AT&T visual voicemail and you lose the significant value of Google Voice (GV).

I’ve run and tested this on 3 unique GV set ups and feel confident it works every time.  You have may have seen and tried other directions online which didn’t work as I did, but working through a friend at Apple and an oddly friendly AT&T tech, this is pretty bullet proof.  Here we go:

1.  Ignore the settings on your iPhone itself.  The master call forwarding feature is useless on the iPhone unless you never want to receive calls on your device and forward right to GV all the time..  Leave the forwarding setting off.

2.  All of the conditional forwarding comes from AT&T Central Office and the standard GSM forwarding codes, however the iPhone doesn’t understand these in native form..  With most GSM phones a single * puts the phone into GSM command mode whereas the iPhone has reserved the first * for its own command lines, so to use GSM commands you must use two.  Hence, when you see the instructions below, all GSM commands start with either ** or #.

(Before you SPAM me, GSM is the underlying command code for EDGE and 3G networks so no matter what iPhone version or network you run on, trust me this works.)

To Forward Unanswered Calls:
1.  Open the Keyboard on the iPhone app
2.  Dial EXACTLY: **61*xxx-xxx-xxxx#  (Again, STAR STAR 61 STAR GV # POUND)
3.  The iPhone screen will darken and provide you feedback that it is now forwarded


To Forward Calls When You Are Busy Or Decline A Call:
1.  Open the Keyboard on the iPhone app
2.  Dial EXACTLY: **67*xxx-xxx-xxxx#
3.  The iPhone screen will darken and provide you feedback that it is now forwarded


To Forward Calls When The Phone is Off Or In Airplane Mode/No Service:
1.  Open the Keyboard on the iPhone app
2.  Dial EXACTLY: **62*xxx-xxx-xxxx#
3.  The iPhone screen will darken and provide you feedback that it is now forwarded

To END All Forwarding Of Calls:
1.  Open the Keyboard on the iPhone app
2.  Dial EXACTLY: #002#
3.  The iPhone screen will darken and provide you feedback that it is now forwarded