busy light

i have a customer that insists on having a busy light when they are on the phone. there are a number of these on the market (they light up when you are on the phone) but the Plantronics one does not work with the ehs adaptor is in use or so Plantronics says. they are using t41's and t46's with Plantronics cs 540 headsets and the ehs 36.

i can't find any product that works with this combination of equipment. does anyone out there know of something that does work?