AT&T MERLIN LEGEND Release 4.0 Feature Reference page 97

Feature Reference
Two types of Callback can be programmed for an extension:
Automatic. Callback is activated automatically whenever the caller
reaches a busy extension or when all trunks in a pool are busy. This is a
status feature that is set to On or Off for each extension.
Selective. Callback is activated only when a caller chooses it by dialing
a feature code or, on multiline telephones, by pressing a programmed
Selective Callback button. On MLX display telephones, a caller can also
select the feature from the display.
With Automatic Callback, when a caller reaches a busy extension or trunk pool,
he or she hears the queuing tone (five short beeps) instead of the busy tone.
This indicates that the system is putting the call into the callback queue.
With Selective Callback, when the caller reaches a busy extension, he or she
must activate Callback while listening to the busy signal. If the caller tries to
make a call by using a pool in which all trunks are busy, he or she hears a fast
busy signal immediately after dialing the pool dial-out code. After activating
Callback, the caller receives dial tone; after all digits are dialed, the caller hears
the queuing tone and the call is added to the callback queue.
With both types of Callback, a caller can either stay on the line until the call is
completed or hang up.
If the caller stays on the line, the red and green LEDs are on next to the
line button used to make the call. When the busy extension or pool is
available, the caller hears the out-of-queue tone (three short beeps) and
the call is completed automatically.
If the caller hangs up, the green LED flashes next to the line button,
indicating that the button is being held for the queued call. When the
busy extension or pool is available, the caller hears a priority ring (four
bursts of ring on an MLX telephone and three bursts of ring on an analog
multiline telephone or single-line telephone). If the user does not answer
the callback call within the number of rings programmed for the system
(1–6), the callback request is canceled.
For inside and outside calls, the caller hears ringback when the extension is
available, but the system does not make the call until the caller picks up.
Considerations and Constraints
Callback cannot be used for personal lines assigned to buttons on a telephone.
See "Line Request" for additional information. If more than one call is waiting for
the same extension or trunk pool, the call that has been queued the longest is
connected first.
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Callback