AT&T MERLIN LEGEND System Reference Manual page 281

Features
Feature Interactions
Auto Dial,
Last Number Dial, and
Saved Number Dial
Paging
Queued Call Console
Transfer
Voice Announce to Busy
System Programming
2-232 Microphone Disable
When an MLX telephone user's microphone is disabled, pressing an Auto Dial,
Last Number Dial, or Saved Number Dial button turns on the speakerphone so
the user can hear the number being dialed. However, the user must lift the
handset to talk once the call is answered.
Calls made to Speakerphone Paging Groups can still be heard over telephones
whose microphones are disabled.
The microphone a QCC cannot be disabled.
Calls can be transferred with a voice announcement to users whose
microphones are disabled. but the users must lift the handset to talk.
Users who are on their telephones and whose microphones are disabled can
still hear the voice-announced call over the speakerphone. They must press the
button the call came in on to talk to the caller.
Enable or disable individual MLX telephone microphones. The factory setting
is enabled.
Extensions -> More -> Mic Disable