A classic example from a business perspective: Poorer users had no money to waste calling people up - but incoming calls were free.

So firms would advertize a missed call number on the radio.

People could give that number a missed call, and they would soon get a call back from the firm - thus never spending any money at all.

The more common interpersonal example was one person having more money than the other, or being on a company plan - so they would get missed calls from their family/friends, and they would then call back.

The cost was thus borne by the firm/richer person/person with more talk time.

Say you want to tell your mom once you safely reach school (via public transport). But you don't actually want to spend the money on a call or text (India did not have "unlimited text", at least when I was growing up there). So your mom and you agree on a protocol that "one missed call means I reached safely". Now you just call her and disconnect immediately. Message received.

Hypothetical Example: I would tell my dad that I'll give a missed call and let it ring just thrice once I got safely to University from home.

Actual Example from my life: My dad had a cellular plan that had lots of free minutes of calling. So, I would just give him a missed call and he would call back. This was the early 2000s I think.

You would tell me "if I am able to make it to the movie tonight I will leave you a missed call around 2pm" and then neither party pays for a telephone call or sms.

We called this “beep”ing in Romania. Apparently it was widespread across developing contries [1]

[1] https://academic.oup.com/jcmc/article-pdf/13/1/1/22316427/jj...