NEC SL1000 Features And Specifications Manual page 351

ISSUE 1.0
Operation
To mute your telephone handset or Handsfree microphone while on a call:
1. Press Mute.
To turn your telephone microphone back on:
1. Press Mute.
Use Mute only if you pressed it initially to turn off your Handsfree microphone.
Features and Specifications Manual
SL1000
M
1-343