Message-waiting indicator

A Message Waiting Indicator (MWI) in telephony, is a Telcordia Technologies (formerly 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.

As described in Telcordia Generic Requirements document GR-283-CORE, a Message_Waiting_Indicator (MWI) is a mechanism that informs the subscriber about the status of recorded messages. The subscriber may subscribe to a notification feature that makes use of the status of this MWI.

This feature is also frequently called (and abbreviated) as Visual Message Waiting Indicator (VMWI). A VMWI, as defined in Telcordia GR-1401-CORE, is a Stored Program Controlled switching System (SPCS) feature that activates and deactivates a visual indicator on Customer Premises Equipment (CPE) to notify the customer that new messages are waiting. VMWI differs from existing features that use other message indicators, such as audible stutter dial-tone, in that it activates a visual indicator on the CPE. The visual indicator may be as simple as lighting or flashing a Light Emitting Diode (LED), or as advanced as displaying a special message on a liquid crystal display.

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.

Use of similar indicator on standalone Caller ID set-top boxes

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). 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.