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.
Voicemail greetings are simple things, and in a business, they can seem unimportant. However, a voice mail greeting gets a lot of play with customers and associates—think of how often you are unable to take a phone call because you're on another line, away from your desk or out of the office.

To set up Visual Voicemail, tap Set Up Now. You’ll be prompted to enter and confirm a voicemail password. Next, you’ll have the option to choose the default voicemail greeting or record your own. If you want to record your own custom greeting, tap Custom. Once you’ve entered created your password and chosen your greeting, you’ll be able to receive voicemails and view them in the Phone app.
As I mentioned earlier, one of the common reasons why an iPhone won’t play voicemails is because the Phone app isn’t working properly. Closing and reopening the Phone app allows it to “shut down” and start again, which can sometimes fix a software glitch.