Why can't I change my voicemail on an updated iPhone?

Voicemail is a carrier service; your iPhone just echoes what is on the carrier’s voicemail server. So first, you must have a cellular network data connection to change anything about email. If you already have cellular data enabled, including for Phone, you have 2 choices:

• Contact your carrier
• Log in directly to the carrier’s voicemail server and change your settings there. Different carriers have different ways of doing this; some common ones: Hold the “1” button on your phone keypad for 2 seconds; Call your own number from your phone; go to the carrier’s website and check for other ways.