AT&T MERLIN LEGEND System Reference Manual page 107

Features
Pools
Queued Call Console
Reminder Service
Remote Access
Speed Dial
Station Message Detail
Recording
System Access
Transfer
2-58 Callback
Callback can be used to complete calls to outside numbers in the Hybrid/PBX
mode only when the call is made by using a pool in which all trunks in the pool
are busy.
Calls to QCCs are not eligible for Callback since the calls ring into the QCC
queue. Callback cannot be used on a QCC.
Reminder Service calls cannot be queued by using Callback.
If the system is programmed for Remote Access Callback, Remote Access
users can use Callback. The user cannot hang up but must wait on the line until
the extension or trunk pool is available. The user hears Music-On-Hold if it is
programmed.
When a Stop character is programmed as part of a Speed Dial number, the user
must stay on the line and wait for the Callback call and then reactivate Speed
Dial. This signals the system to continue dialing the digits after the Stop
character.
SMDR begins measuring the duration of Callback calls when the call is
completed.
Callback can be used on Intercom or System Access buttons. In the Hybrid/PBX
mode, when Callback is used on a Shared System Access button, the Callback
from the system rings and the LED next to the button flashes only at the
telephone that originated Callback.
If a user other than the person originating the Callback selects a Shared System
Access button with a queued call and lifts the handset, the person hears the
queuing tone and the green LED on the originator's telephone goes from
flashing to steady. If the second person hangs up, the green LED on the
originator's telephone goes back to flashing and the system directs the Callback
call to the originator. If the second person does not hang up, the system directs
the call to the second person and not to the Callback originator.
A queued Callback call cannot be transferred, but calls transferred to busy
extensions are eligible for callback. When a user reaches a busy extension
while transferring a call, Automatic Callback or Selective Callback can be used
to queue the call before completing the transfer. The caller hears ringback or
Music-On-Hold as with any transfer.