In many cases, an update to your carrier’s voicemail app or settings can resolve the issue, but don’t forget to call your voicemail number to check if it is set up correctly. Once you’ve set up your voicemail, you’re free to switch off when you need to. There are other ways you can stay in contact, however.
To switch voicemail back on, dial 1211. Record new voicemail greetings. You can record new greetings by calling 121, standard call rates apply.

The telephone systems have the feature named as Voicemail. VoIP is the best example of it. When a receiver is unable to pick up the phone, then the caller can leave the voice message on the receiver’s phone. In older telephonic machines, such messages were stored as a voice message directly, and the receiver can hear them later. In modern telephone systems, such info is being stored on the server of the service provider in the user’s mailbox. It is similar to email in which the messages are saved in the form of audio instead of text.
With Visual Voicemail on iOS 9 you can listen to a Voicemail easily and you can also save or send a message on to someone else through iMessage or email.