I have a Norstar Compact ICS 4.1. Anyway, problem is a call will come in and receptionist will put the call on hold, but then forget that there are callers on hold. The display on all of the phones will flash next to Line X, showing that there is a call waiting on Line X, but no audible tones? I've seen other Norstar systems give a silent ring or chirp when a call is on hold and still waiting to be answered. How do I set this up??