I mean, modems and faxes in effect just whistle into the phone line. At some level of proficiency, you might be able to do that yourself ;) Afaik phone phreaki

I mean, modems and faxes in effect just whistle into the phone line. At some level of proficiency, you might be able to do that yourself ;) Afaik phone phreaking used a variety of whistling devices of different complexity: from https://slate.com/human-interest/2013/02/phone-phreaks-the-toy-whistles-early-hackers-used-to-break-into-the-phone-network.html https://slate.com/human-interest/2013/02/phone-phreaks-the-t... to https://en.wikipedia.org/wiki/Blue_box https://en.wikipedia.org/wiki/Blue_box