NEC ASPILA EX Feature Manual page 272

Operation
258 Features
To mute your telephone's handset or Handsfree microphone while on a call:
1. Press MIC.
This only turns off the Handsfree microphone.
OR
Press Microphone Cutoff key (PGM 15-07 or SC 851: 40).
This turns off both the handset and Handsfree microphone.
To turn your telephone's microphone back on:
1. Press MIC.
Use MIC only if you pressed it initially to turn off your Handsfree microphone.
OR
Press Microphone Cutoff key (PGM 15-07 or SC 851: 40).
Use the Microphone Cutoff key only if you pressed it initially to turn off your handset
or Handsfree microphone.
Microphone Cutoff
Aspila EX Software Manual