What ever your MWI on and off numbers are (Go to Feature->Voicemail-MWI) if the partition in which these numbers belongs to is accessible to phones (via their CSS) then you can dial MWI on / off numbers (say 8001 for Mwi on, 8002 for mwi off) from the phone to turn the light on and off.
HTH
Sankar
PS: please remember to rate posts!
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus