There are a mountain of calling features out there offered by different PBXs. One of these is called busy-on-busy. This is simply a feature that blocks new incoming calls when the user is already on the phone. In a highly responsive and connected UC world, this is less of a desired feature but there are still great use cases for it. In steps the Kuando Busylight. We use these at the top of our cubes at our office so you can see a user’s presence as you pass by, We’ve also deployed these at warehouses where a visual indicator was needed for an incoming call due to the high volume of machinery nearby. To top it off, their latest software allows you to set busy on busy, a feature not native to Lync!
I highly recommend checking them out: http://www.busylight.com/busylight-lync.html