Phreaking
Exploring and fooling the telephone network with tones instead of wires — the forerunner of hacker culture, killed off by signalling moved out of the voice channel.
Phreaking (from phone and freak) was the craft of manipulating the analogue telephone network. For years its control signals travelled in the same channel as the call, and a tone at 2600 Hz made a long-distance trunk believe it was idle — whereupon the phreak could seize it and call around the world for free. The tools got colour names: blue, red and black boxes.
The culture grew up alongside early hacking and the BBS scene; the magazine 2600 is named after the tone. When the phone companies moved their control signalling onto separate channels (SS7) through the 80s and 90s, the simple trick vanished and phreaking passed into history.