Note this: The message waiting light is designed to be able to be lit manually by anyone.
Call an extension, when it is busy or while it is ringing, just press your Message button and you will light their lamp. When they press Intercom and the lit Message buttton it will call you back and stay lit until you have answered. (Imagine for a moment, a company that does not use voicemail but has a receptionist take messages for everyone- that is what it was designed for.)
Cancel it this way: press Intercom 700 plus the ext number that is lit.