Hi
Your problem poses some design problmes:-
As mobile phone could be left anywhere, you cant juz add a proximity or object sensor, a phone left near a book might still think there is someone nearby, so the only way is the intelligently sense if there is a human preset nearby.
One way I can think of is to use the newly introduced Motorola "E-Field" sensor, this sensor uses a very low electric field to sense the presense/absense of a human amongst others.
You might want to google for Motorola "E-Field" or try
**broken link removed**
The chip no is: MC33794
Cheers
Linux