Reset voicemail password from AT&T's automated system

Call 611 or 800-331-0500.
Enter the wireless number starting with area code.
Press 3 to “get help with voicemail.”
Press 3 to reset your VM password.
Enter the Billing Zip code for the account.
After a few seconds, iPhone will display "Password Incorrect - Enter Voicemail Password."
Enter the last seven digits of the wireless phone number and tap OK.

Reset voicemail password from AT&T's website
NOTE: You must be registered and logged into myWireless Account to view this information.

From the myWireless Account home page

, select Phone/Device then choose the Reset Voice Mail PIN link from the left column.
IMPORTANT: If there is more than one line on the account, make sure the correct wireless number is displayed in the Wireless Number list before selecting Reset Voice Mail PIN.
To reset the password for any of the lines on the account, choose a wireless number from the dropdown menu

Click Submit.
After a few seconds, iPhone will display "Password Incorrect - Enter Voicemail Password."
Enter the last seven digits of the wireless phone number and tap OK.

I hope this works for you.  It would great for my coworker.  He even found out that he had 10 voicemails waiting for him on his Iphone.