A message waiting indicator (MWI) is a indicator on a handset, usually a small lamp, that is used to inform the user of the handset when a voice mail message is waiting. The indicator does not have to be implemented as a lamp, sometimes a distinctive dial tone is used instead. The principle is that it is associated with a particular handset, or subscriber number.
The message waiting indicator is set by sending a special message to the switch. To set a message waiting indicator, Blueworx Voice Response sends the SL_STATION_SET_MWI_REQ primitive to the signaling process. Blueworx Voice Response does not know which signaling process is responsible for a given phone number or extension. Therefore the request to set a message waiting indicator is sent to every signaling process installed.
The message waiting indicator works in the same way for both common channel and exchange data link signaling.