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

adaptive answering

    A feature which allows a faxmodem to answer
   the telephone and decide whether the incoming call is a fax or
   data call.  Most Class 1 faxmodems do this.  The
   U.S. Robotics Class 1 implementation however seems not to do
   it, it must be set to answer as either one or the other.

   (1995-03-16)