i know why it goes off but have not yet figured out how to fix it...
if your screen is locked (like when its in your pocket) and you tap the 'hangup' button then a menu appears, the default selection is 'turn off' its no stretch of imagination to think that when about your daily business this button may get tapped a few times, result, phone off.
i need to find a way to remap this button, so this menu wont come up