Comment by Crosseye_Jack

It Depends... Sure you could spam the pager system triggering them one by one, but because how pagers work you could trigger them all at the same time.

Pagers are basically just a receiver of a One-to-Many network. A pager will receive all pages being broadcast as they are "listen only" devices. As the pagers don't talk back to the service provider the SP doesn't know which transmitter to use, so the SP will broadcast the pages out across their whole network. The circuity/software of the pager will then filter out only the messages intended for that pager out of all the pages it receives. To reach pagers out of range/switched off the SP would just repeat the page for a period of time.

(note: for message privacy you can add encryption to the message but back in the day that didn't happen, and you could just pull clear text out of the air.)

This "receiver only" style of device allowed pagers to be low power and would run for a very long time on a single AAA battery (or even on a watch battery, because they built a pager into a watch! The Timex Beepwear... Oh I so wanted one as a kid!). But it has the benefit that because they are one-way/"listen only" you can't track them because they are not communicating back to the mothership! It would be like trying to track an AM/FM radio in a car.

If you are adding an "add-on board" to the device, you could tap the receiver of the pager and do your own decoding of the pages. So you could have the add-on board trigger on a "certain message for this pager only" but you could also trigger on "a certain message sent to pager serial 1234567890".

If you knew the phone number assigned to the pager with the serial 1234567890 (because you just so happened to have paid for service for that pager by what ever clandestine means you wanted) you could trigger them all with a single phone call from a public phone or a disposable cell phone to a pager not even associated to the target group of devices.

EDIT: Just a note to say 2 way pagers do exist, this type of pager allows the pager to confirm receipt of a page and even send their own pages to other pagers, but I would suspect that the type of pager being used in this case is the one-way type pager because its reported they were using them because they are harder to track.