Apple Thoughts: Visual Voice Mail Problems? This May Help...

"This happened to me last time I upgraded from my original iPhone to the 3G. I went to check messages, and instead of seeing the list of people who called, the phone simply dialed the voicemail number. The solution this time is the same as last time."

I never had a problem on any of the upgrades, perhaps I was just one of the lucky ones, but if you have had a problem this may be the fix for you. Basically, go to the AT&T site, log in and under Phone->Device Support, there is an option to "Reset Voicemail Password." Once you do this you will get a text from AT&T with a temporary password. Set up your iPhone voicemail again and use the temporary password, then change back to your original. This is supposed to work but I haven't tried it since I haven't had that problem.