Advisory Message displays short message on the caller’s phone when the user is not available. If you want to add new category or edit the exiting category of the Advisory Message, you need to do that from the Mitel web interface.
From the Mitel configuration menu, go to Users and Devices -> Advanced Configuration -> Multiple Advisory Messages. There you can add or edit the Advisory Message categories.
To use the feature on your phone, press the Superkey (blue button with phone and addressbook icon on your phone), and goto Advisory Msgs section. There you can choose the Advisory Message you want displayed and and press the * key to turn it on your phone. When you have it turn on, the Advisory Message will be displayed on your phone, and the caller to your phone will receive the advisory message displayed on their phone as well.