AT&T System 25 - Manual (Page 18)

Callback
If a System 25 extension or outside line is busy when you place a
call, you can request a callback. Your call will wait in a line
(called a queue) for the extension or line to become available.
The system will then alert you with a priority ring and complete
the call.
For inside calls, use Callback when you hear a busy tone or a
special ringback tone. For outside calls, use Callback only when
you hear fast busy tone.
NOTE: You may hear the callback request queueing tone
automatically when you place certain calls.
Requesting Callback
1 Before you hang up, press
2 Listen for queueing tone, then hang up.
NOTE: If you do not hear
not been accepted for this
Completing Callback
queueing tone, your request has
call.
1 When you hear priority ring, lift the handset.
NOTE: If you remained off-hook after requesting Callback,
you will not receive priority ring.
2 Listen for dequeueing tone.
Your call will be completed as dialed.
Canceling Callback
1 Press
2 Press the button associated with the call.
3 Hang up.
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