NEC SL1000 Features And Specifications Manual page 494

SL1000/SL1100
Operation
To mute your telephone handset or Handsfree microphone while on a call:
1. Press Mute key.
To turn your telephone microphone back on:
1. Press Mute key.
Use Mute key only if you pressed it initially to turn off your Handsfree microphone.
M
1-484
ISSUE 2.0
Microphone Cutoff