The story
From the Demon Dialer product from Zoom Telephonics, Inc., a telephone device produced in the 1980s which repeatedly dialed busy telephone numbers under control of an extension phone.
Etymology from Wiktionary, CC BY-SA 4.0.
demon dialing
Today it means: The use of a computer to dial a number repeatedly (usually to a crowded modem pool) in an attempt to gain access immediately after another user has hung up. Full definition →