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.
A VMWI, as defined in Telcordia GR-1401-CORE, is a stored program controlled switching (SPCS) system feature that activates and deactivates a visual indicator on customer-premises equipment (CPE) to notify the customer that new messages are waiting.
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.
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 two minutes of a new call) to check for a "stuttered" dial tone.
The customer service call center for the mobile network operator may also be able to provide simple technical support to reset this by resending the MWI message manually.