AT&T MERLIN LEGEND Release 4.0 Feature Reference page 538

Feature Reference
Shared SA buttons cannot be assigned to a single-line telephone. A single-line
telephone cannot be the principal extension for a Shared SA button, unless the
telephone is connected to an MFM.
When two or more users answer the same call on a Shared SA button, the red
and green LEDs next to the button go on, but only one person has a talk path
with the caller. Privacy should be used to eliminate competition for the same
calls.
Calls received on DID trunks ring on an SA button and on all Shared SA
buttons for the receiving button.
Mode Differences
Hybrid/PBX Mode
SA buttons, including Shared SA buttons, are available only in Hybrid/PBX
mode.
Key and Behind Switch Modes
ICOM buttons are available only in Key and Behind Switch modes.
Telephone Differences
Direct-Line Consoles
Each DLC is assigned one SA Ring or ICOM Ring and one SA Voice or ICOM
Voice button. Additional SA or ICOM buttons cannot be assigned to a DLC.
This includes Shared SA buttons corresponding to SA buttons on other
telephones. In addition, the DLC cannot be the principal extension for Shared
SA buttons on other telephones.
Queued Call Consoles
A Queued Call Console (QCC), which uses Call buttons, cannot be assigned
SA buttons, including Shared SA buttons (neither can it be assigned ICOM
buttons, since the QCC is available only in Hybrid/PBX mode).
Other Multiline Telephones
In Hybrid/PBX mode, each multiline telephone (except for DLCs) and
Multi-Function Module (MFM) device is automatically assigned one SA Ring,
one SA Voice, and one SA Originate Only button.
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System Access/Intercom Buttons