That is so cool. I really want to get one (or make a hardware company that builds simple, DIY kits for them).
>builds simple, DIY kits for them
So I highly doubt this would be profitable for you.
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As far as the paging infrastructure is concerned, all the messages are mass-sent, in an un-encrypted analogue broadcast. EVERY device receives EVERY page, as long as within range. It is the pager itself which chooses to only display "your" message(s).
Your DIY hardware could lead you into some interesting discoveries of your local area's messaging/users.
Got lots of server health messages and requests to call people back. And some more personal messages, too.
That vibe is reflected in the 9/11 pager messages as well... https://911.wikileaks.org/