My grandma did this back in the 80s here in the US. Back then if you wanted to call someone more than about 10 miles away, it cost money. She lived in downtown LA and we were in a suburb, so it was a toll call.

If she wanted to talk to us, she would call and hang up after one ring. That was our signal to call her. There was no call-ID at the time, so we had to guess if it was her. It still cost us money to call, but she was fine with that!