Darkened Screen

[Bazile]

What's the idea behind the screen going dark when you hold phone to your ear , when making a call on a smart phone?

[Zacs-Master]

So it doesn't waste battery cos you can't see it.

[Tilly2]

So that your face doesn't activate some other icon on your phone.

[Old_Geezer]

I always assumed power saving.

[Gizmonster]

I think Tilly has it, 'cos I always thought that when you put the phone to your ear, your phone detects its close proximity and disables functions on your phone, so you can't accidently hang up by pressing the phone with your ear.