you would call someone person to person and they would say they're not here and you'd say I'll call back at 1:00 and that meant that you were arriving at the airport at 1 so you didn't have to pay for the call?
That reminds me when my brother was in college out of state. Collect calls cost more, so he'd place a collect call then we'd say the person wasn't available. But then we knew he was home and wanted us to call him back direct.