Assuming it the "envelope" button.
If you pick up the handset and press the button and it doesn't do anything you'll need to erase the message waiting.
To do this, put the handset in the cradle, press the MWI button, press the read messages button, then call me back button then the erase button.
If you don't get the erase button, then you'll need to modify your COS to enable "Multiline Set Message Waiting Erase allowed".
Ralph