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?