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Glossary · Voice technology

What is Message waiting indicator?

  • Also called: mwi, voicemail light, stutter dial tone

Reviewed by Sujan ThapaliyaLast updated

Definition

The message waiting indicator is the signal that tells a phone there is a new voicemail — a lamp on a desk phone, a badge on a softphone, or the stuttered dial tone on an older line. It is a subscription between the handset and the voicemail system, not a property of the mailbox.
The three STIR/SHAKEN attestation levels, which underpin how message waiting indicator behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, message waiting indicator included.

In practice

How message waiting indicator actually works

That subscription is why the light is the first thing to break. If the extension registers against a different account than the mailbox belongs to, messages arrive perfectly and the lamp never lights.

It is also why clearing a light sometimes needs the mailbox opened rather than the message deleted: the indicator turns off when the voicemail system says so, not when the handset thinks it should.

On any shared line, voicemail-to-email is a more reliable notification than the lamp, because it does not depend on which handset happens to be registered.

The indicator is also stateful in a way that causes confusion: it reflects what the voicemail system last told the handset, not what is in the mailbox now. Deleting a message from a different device, or from an email attachment, can leave the lamp lit until the mailbox itself is opened.

Worked example

Messages arrive, email notifications arrive, and the desk phone lamp stays dark — the extension is registered against a different account than the mailbox belongs to.

Message waiting indicator: common questions

Why is my voicemail light not working?
The handset is not subscribed to notifications for that mailbox — usually because the extension is registered against a different account than the one the mailbox belongs to. Fix the mapping, then re-register the phone.
What is a stutter dial tone?
The older form of the same signal: a broken dial tone when you pick up, indicating an unheard message. It predates lamps and still appears on analogue lines.
How do I clear a message waiting indicator that will not go off?
Open the mailbox through the voicemail system rather than deleting the message elsewhere. The indicator clears when the voicemail platform sends the update, not when the handset is restarted.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See message waiting indicator in the product

A voicemail drop leaves a pre-recorded message in a contact's mailbox during an outbound call, so the rep moves to the next number instead of speaking the same 20 seconds again. One keystroke, or automatically when answering-machine detection hears a greeting.

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