When the Amber firmware update launched on devices like the Lumia 920, it introduced new features that made it easier and a little more intuitive for users to respond to incoming phone calls. The feature was also available on even older devices such as the Lumia 900 before randomly disappearing.
Good news is that the gesture is back, along with some newer ones too, thanks to the new Gestures Beta app from Microsoft. In addition to silencing the ringer by flipping the phone on a flat surface with the display down, the app allows users to answer incoming calls by picking the phone up and lifting it to their ears, mute active calls by placing the phone screen down on a flat surface, and putting a call on speaking by placing the phone down with the display facing upwards.
The app is available for all Lumia devices running Windows Phone 8 and 8.1, although Microsoft notes that those with the Lumia 530, 630, and 635, will experience limited functionality, probably due to the lack of a gyroscope on those devices.
You can get the new Gestures Beta app by heading over to the download link below. Give the app a try and be sure to use the in-app feedback button to suggest improvements or report bugs.