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

carrier scanner

    (Or "wardialer") A program which uses a modem to
   dial a series of phone numbers (say, from 770-0000 to
   770-9999), and keeps a log of what phone numbers answer with a
   modem carrier.  The results of such a search were generally
   used by people looking to engage in random mischief in
   random machines.

   Since the 1980s, wardialers have generally fallen into disuse,
   partly because of easily available "caller ID" technology,
   partly because fax machines are now in wide use and would
   often be logged as a carrier by a wardialer, and partly
   because there are so many new and more interesting venues for
   computerised mischief these days.

   (1997-03-16)