I have a warehouse that would like to be notified visually when there is a new voicemail sitting in their mailbox. Does anyone know of any physical devices / lamps that could achieve this sort of thing? Basically something that can subscribe to MWI is what I am thinking.
Your best bet is to have an analog line with its message waiting set to that of the mailbox. There are various visual message waiting indicators that then can be placed on the analog line to show the message waiting indicator. If you don’t have analog extensions on your system, an inexpensive analog terminal adapter like the Grandstream HT801 or HT802 can provide the analog port you need.
This is one example of a device for message waiting indication but there are many others: #PAP9Q - Modular Large LED Neon MW/Ring Indicator
This is another from well known telecom equipment supplier Viking:
Seems like a nice Raspberry Pi project.