Acoustic coupler

The Novation CAT acoustically coupled modem
Sendata Series 700

In telecommunications, an acoustic coupler is an interface device for coupling electrical signals by acoustical means—usually into and out of a telephone.

The link is achieved through converting electric signals from the phone line to sound and reconverting sound to electric signals needed for the end terminal, such as a teletypewriter, and back, rather than through direct electrical connection.