AT&T MERLIN LEGEND System Reference Manual page 402

Features
Mode Differences
Hybrid/PBX
Key and Behind Switch
Considerations
In the Hybrid/PBX mode, the system assigns one System Access Ring, one
System Access Voice, and one System Access Originate Only button to all
multiline telephones (excluding system operator positions) and to MFMs. For
DLCs, the system assigns one System Access Ring and one System Access
Voice button. For equipment connected to a 012 or 008 OPT module, the system
assigns two System Access Ring buttons. System Access buttons are not
assigned to a QCC.
Shared System Access buttons can be assigned only in the Hybrid/PBX mode.
In the Key and Behind Switch modes, the system assigns one Intercom Ring
and one Intercom Voice button to all multiline telephones (excluding QCCs) and
to Multi-Function Modules. For equipment connected to a 012 or 008 OPT
module, the system assigns two Intercom Ring buttons.
Shared System Access buttons cannot be assigned in the Key and Behind
Switch modes.
In the Key and Behind Stitch modes, outside calls cannot be made on
Intercom buttons.
Intercom and System Access buttons can be assigned or removed only in
centralized telephone programming.
There is no limit to the number of stations that can be programmed as a
principal station in a Shared System Access arrangement.
The maximum number of Intercom or System Access buttons that can be
assigned to an individual station is ten. A minimum of one Intercom or
System Access button must be assigned to each station.
The maximum number of Shared System Access buttons that can be
assigned to an individual multiline telephone is nine.
On a multiline telephone, Intercom or System Access (including Shared
System Access) buttons are assigned to buttons numbered 1-10.
Each System Access button on a principal station can be assigned as a
Shared System Access button on a maximum of 16 multiline telephones (not
including the principal station).
The maximum number of system users that can be included in a call
(including conference calls) is three.
System Access and Intercom buttons can be set for Immediate Ring,
Delayed Ring, or No Ring.
Shared System Access buttons cannot be assigned in the Key and Behind
Switch modes.
System Access 2-353