First try these instructions:
Reset the voicemail password when using the 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 the voicemail passcode.
Enter the Billing Zip code.
A text message will be sent to the wireless handset. If the wireless account does not have text messaging, please contact Customer Care to have the voicemail password reset.
NOTE: This can take up to 15 minutes.
Or... reset network setting within the iPhone to see if that does the trick. Best luck!!