The Free On-line Dictionary of Computing (30 December 2018):

acoustic coupler

    A device used to connect a modem
   to a telephone line via an ordinary handset.  The acoustic
   coupler converts electrical signals from the modem to sound
   via a loudspeaker, against which the mouthpiece of a telephone
   handset is placed.  The earpiece is placed against a
   microphone which converts sound to electrical signals which
   return to the modem.  The handset is inserted into a
   sound-proof box containing the louspeaker and microphone to
   avoid interference from ambient noise.

   Acousitic couplers are now rarely used since most modems have
   a direct electrical connection to the telephone line.  This
   avoids the signal degradation caused by conversion to and from
   audio.  Direct connection is not always possible, and was
   actually illegal in the United Kingdom before British
   Telecom was privatised.  BT's predecessor, the General Post
   Office, did not allow subscribers to connect their own
   equipment to the telephone line.

   (1994-11-08)