The old school phone network used in-band signaling for call setup/control. If you made long distance calls pre-90s you might remember tones after the call star

The old school phone network used in-band signaling for call setup/control. If you made long distance calls pre-90s you might remember tones after the call started connecting as an example. One of the most significant frequencies in that control setup was 2600hz (which disconnected the call in progress and left you with an open tandem trunk, giving full access to call routing functionality), made infamous by Blue Boxes, Captain Crunch, and the magazine of the same name.