iPhone: How to reset or change voicemail password
Information on how to reset or change your voicemail password on iPhone is available from your wireless carrier.
If you don't remember your voicemail password or need to change it, contact your wireless carrier for assistance.
If your wireless carrier offers Visual Voicemail, you can change your voicemail password directly from the iPhone. Tap Settings > Phone > Change Voicemail Password.
Resetting your AT&T voicemail password
If you are prompted for your voicemail password on iPhone and you do not remember what it is, you can press the ? button in the password field to display an option to call AT&T for assistance. Listen to and follow the menu prompts to reset your voicemail password:
If you need additional assistance, contact AT&T.