4. Reset your pin. Maybe you have forgotten your pin or passcode or you never set it up in the first place. This is a common issue. Most carriers have detailed instructions for changing passcodes. For example, to change a T-Mobile passcode, hold down the "1" key, then press the star key, and then press number 5 to go into passcode security. Then press 1, and you will be able to change your passcode. You can change your Sprint passcode online by choosing the my preferences tab and then the "things I can manage online" section. You can reset your pin by calling the carrier if you have forgotten it or never set it up at all. Some sites allow you to do this online.
If your voicemail has been set up and you are unable to change your greeting through the 'Voicemail' tab, dial *86 then choose option 4 to change. From a Home screen, tap the Phone app . If an app isn't available on your Home screen, swipe left to access the App Library. Tap Voicemail then tap Greeting (upper-left).

5 hours agoIt's not in settings/phone, but in the phone app itself. Bring up the phone as you would for making a call. At the bottom of the screen, you have favorites, recents, contacts, keypad, voice mail. Mclick on voice mail. Now click in greeting upper left corner. …
If you’ve already created a custom voicemail on your iPhone, you can tap Play to see how your voicemail sounds. If you’re interested in creating a new message, tap the Custom button. From here, you can start recording your message by tapping Record and tapping Stop whenever you’re finished. Once you’re finished recording, tap on Play to make sure the message comes across as you’d like and hit Save (in the upper right) to save it as your voicemail.