Acoustic coupler
The modem you pressed a telephone handset into, because attaching your own equipment to the phone line was forbidden.
Telephone companies long owned not only the network but the apparatus, and connecting anything electrical to the line was not allowed. The workaround was acoustic: two rubber cups took the handset, a speaker pushed tones into the microphone and a microphone listened at the earpiece. Typical speeds were 110 to 300 baud, about thirty characters a second.
The Carterfone decision of 1968 opened the American network to third-party equipment and direct-connect modems took over, but acoustic couplers survived well into the 1980s — they were the only way to get online from a hotel room phone or a callbox. The image of a handset jammed into a box remains the symbol of early home computer communication.