AT&T SYSTEM 25 Manual page 28

The Call Waiting feature allows a user at a busy voice terminal to be audibly alerted when
another party is calling. A voice terminal is considered busy if all of its SYSTEM ACCESS
buttons are in use (multiline sets), if it is off-hook (single-line sets), and if all coverage points
are busy.
The called party is notified of a waiting call by one or two short tones heard through the
handset; one tone indicates a waiting inside call, two short tones indicate a waiting (camped-
on) outside call. The caller hears special ringback tone, which is repeated until the call is
answered.
A called party who hears call waiting tones has these options:
Ignore the new call and continue with the current call
Terminate the current call, hang up, and answer the new call when it rings
Put the current call on hold and answer the new call
Call Waiting is enabled or disabled on a per-station basis in system administration, but is not
available for attendant consoles. The call that has been waiting the longest is usually put
through first, but camped-on calls are given priority over other waiting calls. If the waited-for
station dials the Call Waiting pickup code *9 , the first off-hook queued or waiting call will be
answered.
If a station with automatic Callback Queuing calls a busy station with Call Waiting, the calling
station hears queuing tone, not special ringback; furthermore, the called party does not hear
Call Waiting tone. Call Waiting tone is heard only when special ringback is returned to the
caller. A station without automatic Callback Queuing gets special ringback but can manually
queue the call.
A call receiving special ringback can be part of a conference, unless a queued call is already
part of the conference. A waiting call counts as two conferees until it is completed.
Call Waiting
Call Waiting 2-15