The iPhone has a voicemail feature known as visual voicemail. This feature allows you to view a list of messages on your phone and select a message to listen to. In the background, however, your iPhone dials a number to retrieve or access your voicemail. You can change the 9-digit voicemail number from the default number that is assigned to the iPhone.
Turn on your iPhone by holding down the "Power" button at the top of the iPhone.
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Tap the "Phone" icon in the dock of your iPhone. This is a green icon with an image of a phone headset on it.
Tap the "Keypad" in the bottom dock of the Phone menu.
Enter the following: "_5005_86*123456789#." You can substitute the "123456789" with any 9-digit number you want to assign to your voicemail.
Tap the "Call" button and the number is assigned to your iPhone.
Dial the 9-digit number you assigned to your iPhone's voicemail and tap "Call" to make sure you have properly assigned a number to your voicemail. Your iPhone will display "Voicemail" when calling the number.