We use for productivity purposes the Luxafor light (www.luxafor.com) in which you can choose if someone may disturb you or not. I thought that it would be a very handy feature to let the light also be used as an indicator if someone is calling. The source of the Luxafor light is open and they encourage programming to the software. They also have an API available. I think this will be a reason to purchase 3CX, especially in bigger environments.