When they create new models of phone, they don't always use the same type of sensors, or in the same locations. Just because you don't see them doesn't mean they're not there. Try making a call, and while on the call, put your hand over the face of the phone (trick the phone to thinking you put it up to your head.) If the screen goes black... And then you take your hand away and it goes non-black again, that means the proximity sensor is in fact working, even though you can't see it.