the s the novel airport by arthur hailey makes reference to a selcal system depicted in the movie to show the alert about mrs quonsett and then a second alert about d o guerrero that allows an individual aircraft to be signalled so that a private message can be transmitted without other aircraft being allowed is this a precursor to the acars system i assume it used some of the same technology as vhf mobile telephone selective call signalling systems the aircraft's phone number was dialed on a frequency and the aircraft compared the incoming number to the one in its own memory circuit if it matched it signaled with a bell or other such device and presumably signaled the ground that it was aware of the page did each airline have its own frequency or was it a common frequency that protocol did not allow an airliner to listen into unless it was signalled to listen