The 1960s The novel Airport by Arthur Hailey 1967 makes reference to a SELCAL system depicted in the 1970 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