AT&T MERLIN LEGEND System Reference Manual page 249

Features
Feature Interactions
Auto Dial
Automatic Line Selection
Barge-In
Callback
Conference
2-200 Headset Options
When a telephone or console has a Headset Hang Up button, Headset
Status is needed only to manually activate and deactivate headset operation
when the user wants to switch between using a headset and using a
handset.
If a user with active Headset Auto Answer presses the button with a ringing
call, the call is answered without the user hearing the tone.
Headset Auto Answer can be activated and deactivated during a call without
disconnecting the caller and takes effect immediately.
A user with active Headset Auto Answer must manually select a line button or
Call button before making an inside or outside call.
A user can press the Speaker button to move the call from the headset to the
Speakerphone.
When two or more users answer the same call on a Shared System Access
or Personal Line button, the red and green LEDs next to the button go on, but
only one person has a talk path with the caller. The use of Privacy eliminates
competition for the same call.
Headset Auto Answer does not automatically answer voice-announced calls.
When a user does not have a headset, the handset microphone can be
turned off only when the user lifts the handset.
When a caller hangs up, the call is automatically disconnected and the use
of the Headset Hang Up button is unnecessary.
If headset operation is activated on the telephone or console, the user must
select a line button before dialing an extension or an outside number using Auto
Dial.
Automatic Line Selection does not work when an MLX telephone or console is in
headset operation. A headset user must select a line manually before making a
call.
If Barge-ln is used to contact a user with Headset Auto Answer, the call is
automatically answered.
Callback calls are answered automatically using Headset Auto Answer, but the
user hears the Callback dequeuing tone instead of the Auto Answer tone. When
both tailing and receiving users have headsets with Headset Auto Answer
activated, the person being called hears zip tone when the Callback call is
completed but the Callback originator does not hear zip tone or dequeuing tone.
Headset Auto Answer is disabled and must be reactivated manually while an
MLX telephone user with a headset is setting up a conference.