If you are unavailable or busy and cannot receive all of your calls, then voicemail is the quickest way for people to leave messages. Also, with a variety of message notification options available (email alerts, text messages and telephone notification), you can keep up-to-date with any important calls and messages.
Voicemail systems are designed to convey a recorded audio message to a recipient. To do so they contain a user interface to select, play, and manage messages; a delivery method to either play or otherwise deliver the message; and a notification ability to inform the user of a waiting message. Most systems use phone-networks, either cellular or land-line based, as the conduit for all of these functions.