Since fw 10.1.33.33 "Listen-in" is a new feature for models D715, D725, D745, D765, D315, D345, D375, and D385. It allows a second person to listen in to an ongoing call on the headset, by lifting up the handset. This comes in very handy in call centres, where the supervisor would like to listen, but not speak in a call, for training or quality assurance purposes.
This function makes it possible to listen on the handset in addition to the headset, without the caller being noticed.
To use this function, the setting listen_in_on_handset needs to be turned on.