HTC One M9 - Manual (Page 32)

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 &
notification > 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 M9 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 & notification, 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 M9 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 &
notification, and then select or clear Pocket mode.
32 Your first week with your new phone