Same issue here, but on a iphone 4 upgraded to iOS 5.
I have spent a couple hours trying all tips on the web (reset network, reset vm pw, etc) without success.
Note that I am *NOT* getting the "incorrect vm password" pop-up...when I touch the Voicemail icon, it simply dials the VM number, like on an old school dumb phone.
Thanks. That is indeed what was required.
In fact, I am convinced that every other "trick" is a waste of time...just call your carrier (AT&T in my case).
ASIDE: Since there is a lot of AT&T bashing online, I'm happy to report that I got *outstanding* service when I called AT&T today. I phoned 611 a few minutes after phone support opened on a Monday morning, had only about 3 minute wait and the agent was totally clued-in on how to solve my problem.
I've had my share of of "below-average" support from AT&T, but this time it was really good.
FOR SPRINT: If you are trying to get visual voicemail to work and you are on an Airave it won't work. I found out the hard way. Multiple restores and hours upon hours of phone time with the tech department for nothing.
1) If it says your mailbox is not valid, call and have them reset it.
2) Set up your mailbox, then leave yourself a voicemail.
3) Go somewhere there is an actual Sprint signal and wait.
4) Enjoy visual voicemail.
While you are on the Airave signal you will not get voicemail notifications. When you leave and get a signal they will pop up. Good luck.