Originally Posted by fury
If you have it on speakerphone or are using a headset (wired or wireless), it will go to sleep when you hit that button, rather than hanging up the call. When you're using the handset (i.e. the phone mic and the earpiece above the screen), it automatically shuts off the display and touch sensor when you have it close to your face, so there is no battery life benefit gained from hitting the sleep button. That's why they made it hang up the call in that case, because the phone already minimizes power when you're using the handset.
Good to know fury! Thanks