Message Waiting Indication (MWI) is a service feature ofISDN that alerts the calling party that a message
from a calling party is present. The MWI service feature is available in telephones, PBXs and private branch exchanges and is indicated by a flashing visual indicator. As soon as a voice message or a text message is received in the voicebox, the visual MWI indicator is activated by the network operator. The call number of the voicebox must be stored in the telephone system. After retrieving the message, the network operator deactivates the Message Waiting Indication, which then goes out.