Lucent Technologies MERLIN Plus Release 2 System Manual page 198

HOW TO USE
If you have a HFAI button on your voice terminal and you want your intercom
calls answered automatically:
1 Touch HFAI or touch HFAI/Mic.
The green light flahes on BIS-10 and 10-Button HFAI models when active on a call and
remains steady on all other BIS models.
H F A I /
Mic.
MERLIN Plus System Features 5-79