Message Waiting Indicator
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Message Waiting Indicator (MWI) in telephony, is a Bellcore term for an FSK-based telephone calling feature that illuminates an LED on select telephones to notify a telephone user of waiting voicemail messages on most North American public telephone networks and PBXs.
This technology was rolled out in 1995, with the introduction of CLASS-based calling features and ADSI. It was at one time, only compatible with ADSI-compliant telephones but is now compatible with any CPE that simply responds visually to visual FSK.
Note: This service is often erroneously associated with the abilities of most Caller ID standalone set-top boxes. Caller ID boxes manufactured after 1998, feature an LED that blinks green to notify that new calls have been recorded and red to indicate that a subscriber has new voicemail messages waiting. Some units will also display the text "MESSAGE WAITING" (similar to ADSI-compliant telephones). It should be noted that these units do not use Visual FSK to activate their red LEDs, but instead, they briefly "pick-up" the line at certain intervals (normally, within 2 minutes of a new call) to check for a "stutter dial tone." The presence of a stutter dial tone will activate a red LED; while absence will de-activate it.
