How do I access my iPhone voicemail from another phone?

You can access your iPhone voicemail from any phone by dialing your iPhone's number from the other line. After calling your number, to access your voicemail:

On AT&T:

Interrupt the voicemail greeting by pressing the star key (*).
Enter your password (the default password is your 7-digit phone number without the area code).

Interrupt the voicemail greeting by pressing the pound key (#).
Enter your password followed by the pound key (#).