[intended]

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.