Some carriers require you to reset the voicemail password as a security precaution when you get a new iPhone. Sometimes, updating it manually online or by calling customer support can reset your iPhone’s connection to the voicemail server and fix the problem.
6. Return to the dialer and enter the number from Step 6 (along with the *) once more.

If you still remember the password of voicemail on iPhone, you can easily change the password of your voicemail on iPhone. When you forget the voicemail password, you should retrieve the information from the Email or call the carrier directly for a solution.
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.