Old-school BBS term. In the days of dialup bulletin board systems, most of them were single-line, which meant that during prime-time 5-11pm, there would be a few dozen users trying to get on all at the same time, where each one would stay on for half an hour to read and reply to that day's messages and possibly playing some BBS Door games.
ATDT 5143642937
BUSY
ATDT 5143642937
BUSY
ATDT 5143642937
BUSY
... and so on.
Ramdialing is the feature of
terminal emulator software auto-
redial:
dial,
busy signal,
hangup,
lather, rinse, repeat until it gets the line... and hopefully being there to catch it.