VT 9500 - Headset Speaker Volume Button; Headset Mute Button

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4.2 Headset Speaker Volume Button
The speaker volume buttons: to adjust the headset speaker volume.
4.3 Headset Mute Button
Mute button: to make the microphone mute or on.
Speaker volume button＂+＂
Speaker volume button”-＂
Mute button