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Maybe I’m living in the 1970s but I always assumed pagers just showed a phone number and you were supposed to call back?

No, you can definitely send a text message to them, they just don't have the ability to send anything back.

the most basic pagers can only receive callback number. More advanced ones have additional features to display text messages as well.

Except for the 2-way ones

What next? 2-way voice communication!? ;)

If you can send a sequence of digits, you can also send a sequence of letters (a text message) using the same technology.