HTC One E8 - Manual (Page 41)

Phone Basics 28
Three-finger Touch
While using the HTC Car app, touch the screen with three fingers to activate the voice command
mode.
Motion Gestures
Use motion gestures to rotate the screen, mute, lower the ringtone volume, and more.
Flip to Mute
When a call comes in, you can turn your phone over to mute it.
In Settings, touch Sound > Flip to mute to set your phone to:
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Mute once. Mute only once after you flipped the phone. The phone will ring again when there
are subsequent incoming calls.
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Mute always. After flipping the phone to mute a call, keep it always muted for all incoming calls.