AT&T System 25 - Manual (Page 9)

Handset Tones
System 25 signals you with tones that are heard through the
handset. The tones and their descriptions are listed below.
DIAL TONE (a continuous steady tone)
You can dial.
CONFIRMATION TONE (three short tones)
Your action has been accepted.
RINGBACK TONE (repeating on/off tone)
The number you dialed is ringing.
SPECIAL RINGBACK TONE (one long tone and
one short, dull tone)
The person you dialed
Waiting.
is busy, and has been notified of Call
BUSY TONE (repeating on/off tone)
The number you dialed is busy.
FAST BUSY TONE (fast repeating on/off tone)
The number you dialed was busy, misdialed, or restricted.
CALL WAITING TONE (short tone(s) heard during a call)
(the waiting call is an inside call)
(the waiting call is an outside call)
You have a call waiting to be answered.
QUEUEING TONE
(five short tones)
Your request for a callback has been confirmed.
DEQUEUEING TONE (three short tones)
The extension or outside line that you requested for callback is
now available.
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