Acoustic coupler modems were an artifact of the Bell Telephone monopoly and it ability to forbid connecting their wires to non-Bell equipment. Once they were broken up, and we could connect to the wires directly, nobody ever used an acoustic coupler again.

Decades on, the fragments of Bell have been linking up again as regulatory capture and right-wing politics have neutered regulation of monopolistic practices. Now we have ATT and Verizon throwing their weight around.