Instead of being able to listen to avoicemail directly on their iPhone, theVoicemail section of the Phone app instead prompts users to"Call Voicemail." An Apple support document suggestsresetting your Networks Setting to return to VisualVoicemail. You can do this by navigating to Settings> General > Reset. How do you change your voicemail number?
However, some cellular service providers allow users to access and update their voicemail by calling their own numbers from working landlines or phones on other wireless networks, so users whose accounts include such an option and who have access to alternative service could keep the voicemail greetings updated even when their own mobile phones cannot connect to cellular or internet service.

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.
4. Before each voicemail message is played, enter the number associated with deleting a message.