Kuando Busylight Combi - for phones and headsets. Connects to your desk phone and headset/handset, Automatically displays a red light when in a conversation, Manual switch turns light to 'Blue' for Do not Disturb' mode. Headset in use - lights up red when using the headset. Do not disturb - Lights up blue (manually) when you need to focus on a difficult task. The kuando Busylight Combi is a polite, effective way to let people know you are in a call or need to concentrate without being interrupted. kuando Busylight Combi is compatible with 90% of all desk phones and headsets. It is not compatible with Softphones and USB headsets It is not compatible with Mobile Phones.