Its just a thing that they do, mine does the same but im on O2.
After 15secs of not using it, the light will go off, about a minute later, the lights come back on then goes onto a O2 logo screen.
After that, the lights go off after 15sec or something, and then after a minute again, it goes onto the screen that shows the time, and what profile you have set, and if the keypad is locked etc.