In this day of IoT, this could be a FUN project Martin Flynn offered
Waveware SPS-5 transmitter with 4 pagers.
That should be perfect! Pagers are retro. 4 of them mean some will always be charged and ready. But the master station needs serial input from something. Let's see - a pushbutton to the Raspberry Pi running the overhead video display, which ought to have motion and distance sensors to react to folks automatically. The "call button" could respond with a video - the cavalry is coming! Or one of a random assortment of science fiction talking computers such as the talking head from The Starlost or Red Dwarf. - the big red button on the Cray - setting the PiDP-8 console switches all "on" - the Apple ][ serial port card. Evans' Lego robot could entertain them while waiting. For movie trivia, have it say "My name is Rags! Woof woof woof" :-) -- jeffj