HTC ONE User Manual page 36

Flip to Silence
When a call comes in, you can turn your over to mute it.
Pick Up to Lower Volume
In a business meeting or restaurant, pick HTC One up when a call comes in to lower the
ringtone volume.
Set Volume to Automatically Increase while in Your Pocket or Bag
You can set HTC One to recognize when it's in your bag or pocket and raise the ringtone
volume so that you can hear it in noisy environments.
Phone Basics 21