TL;DR: If you replace an iPhone with a non-iPhone on the same AT&T account, make sure AT&T “re-features” your account voicemail, whether you want to use it or replace it with Google Voice.
I recently replaced my first-gen Apple iPhone with a Motorola’s new premium Android phone for AT&T, the Atrix 4G. Some time ago I switched voicemail handling from the iPhone’s “visual voicemail” to Google Voice. It turned out to work very well for me so I stuck with it.
Naturally I wanted to continue using Google Voice when I upgraded to the Atrix, so I didn’t change a thing when my service was transferred. Eventually I discovered that unanswered calls to my Google Voice number would go to voicemail, but calls to my old carrier-assigned number would ring forever (or until the caller lost patience and terminated the connection).
Attempts to correct this by setting Google Voice as my voicemail handler with the controls in the Google Voice web application (and the given MMI codes) and adjusting the Call Settings on the phone met with cryptic error messages.
Eventually, a web search provided the important nugget of information that led to a resolution. The voicemail feature on my AT&T account had a lingering setting for iPhone visual voicemail that was preventing manipulation. A few minutes in chat with an AT&T rep had that cleared up!
This may be important to others as well: I made a good number of frustrated attempts to call my carrier-provided phone number and kept hearing the endless ringing, only to find a voicemail from my brother waiting for me when I returned to the computer and phone after walking away for a few minutes. I’d been testing by calling from the only other available phone I had: Google Talk. And yes, Google Talk uses my Google Voice number, so I was just being forwarded back to myself.