This is happening a bit now, and I don't believe it to be HTC one related, but you'll be talking along and a brief maybe 1-2 second part of the conversation will be covered up with the sounds of touchtones, like 4 buttons being pressed at once. I know I'm not touching the screen, as it's even happened with my bluetooth headset. I suspect it's what happens when it switches to a roaming tower, something my One wouldn't do until an update a few months ago.