Dell Pro Stereo Headset UC300-Lync Optimised - Manual (Page 16)

16 │ Setting Up Your Headset
Adjusting the Volume
Use the “+” and “–“ button located at the center of the call control unit
to adjust the volume. Press “+” to increase the volume level and “–” to
decrease the volume as shown below.
Muting/Unmuting a Call
Press the mute button to mute and un-mute the microphone as shown
below. Whenever the microphone is muted, a red LED glows indicating that
the microphone is muted, and whenever the microphone is un-muted, the
red LED stops glowing.