Nexus 4 & ATT postpaid = settings for google voice (working)

Just received Nexus 4, it is awesome, battery life is OK so far, not much idea but it is on fine side (not different than iPhone 4S)

Tried setting up Google Voice voicemail by dialing code that is generated by google voice (in browser) and it didn't work. But I tried setting it up via "call settings" in the dialer and selected voicemail service as google voice (it is listed in there and default being ATT/Carrier) and it worked.

Funny, I have an HTC Vivid with ATT and had the exact opposite experience.