This button transfers the sound input and output from the handset to the ambient microphone and loudspeaker.
There is a very definite, noticeable switching action each time the phone "changes directions" and a cough or other transient noise in the room may interrupt incoming sound from the far end of the telephone connection.
Full-duplex speakerphones are able to transmit and receive simultaneously and there is no discernible change of transmission direction.
During this calibration phase, they may make a variety of noises including ascending tones or chiming sounds.
This calibration may be automatic or manually invoked by the user, and it may need to be repeated if the speakerphone is relocated within the room.