To turn off the device, you need to hold down the button identified with the red handset until the battery LED (the first from the left) changes from green to red.
The battery LED sequence will be this: by pressing the button with the red handset, the battery LED will turn green, then it will turn off and then it will turn red. At this point you can release the button, held down for the entire time, and the device will turn off after a few seconds.
