AT&T MERLIN LEGEND System Reference Manual page 247

Features
Telephone Differences
Multiline Telephones
MLX Telephones
2-198 Headset Options
To use Headset Auto Answer, Headset/Handset Mute, or HeadSet Hang Up on a
telephone with a Headset Status button programmed, the user first presses the
Headset Status button to activate headset operation (the green LED next to the
button goes on).
When the headset operation is activated, the switchhook operation for a handset
is disabled and the headset can be used to make and answer calls. The user
deactivates headset operation and switches back to the handset by pressing
the Headset Status button again (the green LED next to the button goes off).
Headset Status must be programmed on a button and can be used only on MLX
telephones, digital DLCs, and QCCs.
Note: The user should deactivate headset operation before removing the
Headset Status feature from a button. If headset operation is active, the user
exits telephone programming prematurely.
Headset Options cannot be used on analog multiline telephones.
Headset Auto Answer
Before Headset Auto Answer can be used, headset operation must be activated
either automatically following centralized programming of a Headset Hang Up
button or manually by using a Headset Status button. Once headset operation is
active, the user presses the programmed Headset Auto Answer button. The
green LED next to the button is on when the feature is active and off when the
feature is not active. The user cannot use feature codes to activate or deactivate
Headset Auto Answer. MLX display telephone users can select the feature from
the display only during programming.
Headset/Handset Mute
Before Headset/Handset Mute can be used to turn the headset microphone off,
headset operation must be activated either automatically following centralized
programming of a Headset Hang Up button or manually by using a Headset
Status button. Once headset operation is active, the MLX telephone user turns
the headset microphone on and off by pressing the programmed
Headset/Handset Mute button. To turn the handset microphone on or off when a
headset is not used, the user presses the programmed Headset/Handset Mute
button.
The red LED next to the button is on when the headset or handset microphone is
off and off when the headset or handset microphone is on. The user cannot use
feature codes to activate or deactivate the feature. MLX display telephone users
can select the feature from the display only during programming.