If I'm not off track, the code you asked about is in Feature Access Codes. Message Waiting - Deactivate. You can also Activate the light on if you like to watch it flash or just to test if its working.
On my system its #90 to deactivate, so at the station, leave handset on hook, I punch in #90 6123 (ext number) . MW Light will go out.
I've had this on a couple systems I've worked on.