When I was a student (around 1998/99) I had to buy a mobile phone, but could not afford a subscription.

Fortunately we only pay for outgoing calls in my country, otherwise it would have been too expensive.

I bought a prepaid sim card, that had an expensive cost per minute (almost $1). I would call my parents landline once, as a signal to call me back.

It's one of the main reasons why SMS was the preferred communication methods between students, because calls were too expensive. This habit continued when we grew up, and now almost nobody calls each other. I guess the phone companies didn't think of that scenario...