The present ETS specifies the stage 2 description of the Supplementary Service Call Waiting (SS-CW) for the Terrestrial Trunked Radio (TETRA). SS-CW permits a called user to acknowledge an incoming individual call while he is already busy. Subsequently that user shall have the choice to accept, reject or ignore that incoming call. Man-Machine Interface (MMI) and charging principles are outside of the scope of this ETS.