Here is a strange one:
The MWI is on for a particular extension, however, there is no subscriber for this extension.
I've run a report from unity tools for Alternate extensions that contain xxxx, and for MWI extensions containing xxxx both of which came up 0
I've looked for the corporate directory, removed the phone completely still the light is on.
the code on might have been dialed from that phone if it is not locked down. find out what the code is for lamp off and dial it on the phone directly to turn the light off.