[wyldfire]

I didn't know this was the case. Interesting!

From [1]

> It was not until a landmark U.S. court ruling regarding the Hush-A-Phone in 1956 that the use of a phone attachment (by a third party vendor) was allowed for the first time; though AT&T's right to regulate any device connected to the telephone system was upheld by the courts, they were instructed to cease interference towards Hush-A-Phone users. A second court decision in 1968 regarding the Carterfone further allowed any device not harmful to the system to be connected directly to the AT&T network. This decision enabled the proliferation of later innovations like answering machines, fax machines, and modems.

From [2]:

> After the ruling, it was still illegal to connect some equipment to the AT&T network. For example, modems could not electronically connect to the phone system. Instead, Americans had to connect their modems mechanically by attaching a phone receiver to an acoustic coupler via suction cups.

[1] https://en.wikipedia.org/wiki/Acoustic_coupler

[2] https://en.wikipedia.org/wiki/Hush-A-Phone_Corp._v._United_S...