HTC One M8 - Manual (Page 30)

Flip to mute
If your phone is facing up when a call comes
in, turn the phone over to mute it.
If you go to Settings and then tap Sound >
Flip to mute, you can set your phone to:
Mute
once Mute only once after flipping the
phone. While the phone is still
facing down, it will ring again
when there are other incoming
calls.
Mute
always After flipping the phone to mute a
call, keep it always muted while
it's still facing down when there
are other incoming calls.
Pick up to lower volume
In a business meeting or restaurant, pick
HTC One up when a call comes in to
lower the ring volume automatically.
You can turn this feature on or off. Go to
Settings, tap Sound, and then select or
clear Quiet ring on pickup.
Increase ring volume automatically while phone is in your pocket or
bag
Pocket mode makes HTC One recognize when your
phone is in your bag or pocket and raise the ring volume
so that you can hear it in noisy environments.
You can turn this feature on or off. Go to Settings, tap
Sound, and then select or clear Pocket mode.
30 Your first week with your new phone