This seems so stupidly old school, but the whole world used to run on these things called “pagers” or “beepers.”
Circa 1997 it was common for people to carry around these big fat chunky plastic devices that would display a phone number on it.
You would then find a landline phone to call back that number.
However people got clever and started making codes they could send using roughly 12 characters or less.
This LA Times article from 1997 showing how teenagers would use these codes to communicate
It’s funny to think how “high tech” this was ~20 years ago. Compare this simple one-way communication device showing 12 characters compared to a regular modern day iPhone!