AT&T MERLIN LEGEND System Reference Manual page 405

Features
Direct-Line Console
Disallowed Lists
Display
Do Not Disturb
Forward and Follow Me
2-356 System Access
Once answered by a receiver, the call is removed from the sender's Intercom or
System Access button (including all Shared System Access buttons). However,
when a calling group is programmed as a Group Coverage receiver, the call is
removed from the sender's telephone as soon as it is sent from the calling group
queue to an available member.
In the Hybrid/PBX mode, if a receiver programs a Primary Cover, Secondary
Cover, or Group Cover button programmed for a sender and also has a Shared
System Access button associated with the sender, the green LED next to both
the Cover button and the Shared System Access button flashes. Additionally,
the red LED stays on steady at the Shared System Access button.
Shared System Access buttons cannot be assigned to DLCs.
In the Hybrid/PBX mode, a user cannot use a System Access button (including
Shared System Access buttons) to dial an outside number when the number is
on a Disallowed List assigned to the telephone.
In the Hybrid/PBX mode, if a user calls an extension and the call is answered at
a Shared System Access button, the display shows the principal's extension and
not the answering extension.
Do Not Disturb prevents ringing of incoming calls at Intercom or System Access
buttons (including Shared System Access buttons) on the telephone where the
feature is activated. In the Hybrid/PBX mode, this also prevents calls received
on the principal's System Access button from ringing at other telephones with
Shared System Access buttons.
In the Hybrid/PBX mode, calls are forwarded from the telephone that activated
Forward or Remote Call Forward. A Shared System Access button cannot be
used to activate Forward or Remote Call Forward for the principal's telephone.
Calls received on a Shared System Access button are not forwarded.
When calls are forwarded to an extension number, a call received on an
Intercom or System Access button rings once at the forwarding station's
Intercom or System Access button (including all assigned Shared System
Access buttons even though a call received on these buttons is not forwarded)
and also rings at the destination station's intercom or System Access button
(including all assigned Shared System Access buttons).