>> It was illegal to connect any equipment beside Bell's equipment to the US telephone system. Not only would you be disconnected and possibly fined by your phone provider for doing so, but American law also made it illegal to sell such devices as well for use on the phone network.

Which is the exact thing which gave rise to phone phreaking and getting around the limitations on Bell Systems.

"Exploding The Phone" by Phil Lapsley is a great book that examines these early hackers:

https://lareviewofbooks.org/article/the-invisible-playground...