AT&T MERLIN LEGEND System Reference Manual page 401

Features
Shared System Access
Button
2-352 System Access
A Shared System Access button, available only in the Hybrid/PBX mode, is used
to allow two or more users to answer each other's calls, join conversations, or to
make or receive inside and outside calls on each other's System Access Voice
or System Access Ring buttons.
In a Shared System Access button arrangement, the principal (or primary)
station is the telephone from which Shared System Access buttons are
assigned. Each Shared System Access button corresponds to a specific System
Access Ring or Voice button on the principal station's telephone. The green LED
next to a Shared System Access button flashes when a call is received on that
specific button and is on steady when the button is in use either at the principal
station or on a Shared System Access button.
Shared System Access button assignment is useful when a small group of
users—for example, a boss/secretary arrangement or a small group within a
system that receives many incoming calls on DID trunks—needs to answer and
distribute each other's calls easily or be able to join conversations. To provide
complete coverage within the group, each System Access Ring or Voice button
assigned to a principal station must be assigned to each member of the group.
One System Access Ring or Voice button can be assigned as a Shared System
Access button on a maximum of 16 telephones (not including the principal
station).
A Shared System Access button is factory-set as a Ring button but can be
changed to a Voice button. When a Shared System Access button is set to Ring
and is used to make an inside call, the person called hears the telephone ring.
When a Shared System Access button is set to Voice and is used to make an
inside call, the person called hears the caller's voice instead of the telephone
ring.
When a Shared System Access button is used to make an outside call, the user
selects an outside trunk by dialing either the code for ARS or the pool dial-out
code.
A Shared System Access button can be used to join an in-progress
conversation. The maximum number of people that can join a call is two (a
maximum of three system users for each call).
Shared System Access buttons can be assigned only in centralized telephone
programming. Each multiline telephone can have a maximum of nine Shared
System Access buttons, and at least one System Access button must be
assigned. A telephone can have only one Shared System Access button for
each System Access button on the principal station.
Shared System Access buttons cannot be assigned to single-line telephones or
other tip/ring equipment connected to a 012 or 008 OPT module. Shared System
Access buttons can be assigned to a tip/ring or external alert device connected
to an MLX telephone by an MFM.