If this is a residential application, you are probably not getting the type of message waiting voltage from the phone company that the LM-1A needs.  You probably have “stutter dial tone” provided by the phone company when there are messages waiting.   If you get a broken dial tone when you pick up a phone to make a call when you have messages waiting and a steady dial tone when there are no messages waiting, this is “stutter dial tone”.  The LM-1A is not designed to detect “stutter dial tone” but will flash on incoming calls.  The model you need for “stutter dial tone” detection is a VR-1B.