I've done this, too. Only I would whistle. Start low and gradually rise. Eventually I'd hit the right frequency. The application was when I answered the phone

I've done this, too. Only I would whistle. Start low and gradually rise. Eventually I'd hit the right frequency. The application was when I answered the phone and heard nothing. Sometimes it was a spurious call from a modem. Maybe a wrong number. Maybe a friend thinking I'd be available for a chat (houses usually had just one phone line back then). BTW, I'm not sure I quite believe the story from the post. If my memory serves me correctly, it is the answering modem that would produce a tone first.