External Busy Lamp

[KURT]

I have a customer that wants an external busy lamp for his call center phones. The users sit in cubicles and he wants to see when they are on a call. He needs to see them when he walking around the office so a sidecar on his phone will not be adequate.
I can install an external light above each cubical so they can be seen but need a way to turn them on when user in off hook. They all use headsets so I will not be able to use the hook switch.
I have a FXS gateway that I can use to turn a light on if I can get an analog station to go off and on hook when the user does. Is there a way to make an addition station go off hook when there IP phone does or another way to do this? They all use T54W phones.