VMWI
Visual (or Voice) Message Waiting Indicator. An FSK message sent without power ringing to a CPE to activate an LED or other visual indicator on the receipt of voice mail at the local phone company.
Message Waiting Indication (MWI) comes in two forms:
The first is a stutter tone when you pick up the phone that some people say sounds like a busy signal. The stutter tone will not interfere with the normal operation of the voice service, it is simply a reminder that you have voicemail waiting for you.
The second form of MWI is Visual Message Waiting Indication or VMWI. WMWI is handled differently by each brand of phone. Some phones show a picture of an envelope on the display. Some phones have a flashing or solid light to indicate a message is waiting.
Some phones have neither, but will briefly ring to indicate that a voicemail is waiting.