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 introduced 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 Customer Premises Equipment (CPE) that simply responds visually to visual FSK.
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 "stuttered" dial tone." The presence of a stutter dial tone will activate a red LED; while absence will de-activate it.
