AT&T MERLIN LEGEND System Reference Manual page 106

Features
Group Calling
Headset Options
Hold
Line Request
Multi-Function Module
Music-on-Hold
Paging
Park
Personal Lines
Pickup
Calls made to a calling group are not eligible for Callback since the call rings
into the calling group's queue. However, Callback can be used for calls to
individual calling group member extensions or to the delay announcement
device. Calling group calls are not sent to the group member telephone when
the calling group member uses Callback for a busy extension or pool, or if
another person used Callback to reach a calling group member and the
Callback call is ringing on that person's telephone.
Callback calls are answered automatically by using Headset Auto Answer, but
the user hears the Callback dequeuing tone instead of the Auto Answer tone.
When both calling and receiving users have headsets with Headset Auto
Answer activated (MLX telephones only), the person being called hears zip tone
when the Callback call is completed but the Callback originator does not hear
zip tone or dequeuing tone.
If the user with Headset Auto Answer uses Do Not Disturb, Callback calls
override Do Not Disturb and are automatically answered.
Pressing the Hold button while waiting for a queued call is the same as hanging
up-the call is disconnected. If a system operator presses another line button,
an Auto Dial button, or a DSS button while waiting on the line for a queued call,
the call is disconnected-Automatic Hold does not work.
Returning Callback calls cancel Line Request.
Both Automatic and Selective Callback can be used from an MFM; however, a
callback call cannot be manually canceled.
An outside caller waiting in the Callback queue hears Music-on-Hold.
A speakerphone paging (voice-announced inside) call that is queued via
Callback automatically becomes a ringing call. Callback cannot be used for
calls to a speakerphone paging group. Systems with loudspeaker paging can
be setup to allow calls to be queued for the loudspeaker paging system by
placing the loudspeaker paging port in its own pool and having users access
the paging system via the pool. When the pool is busy, the "call" can be queued.
Calls waiting in a Callback queue cannot be parked.
The Callback feature cannot be used to request a busy personal line.
A Callback request cannot be picked up at another telephone.
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