If you are using screen guard on you iPhone 4 and your face is activating touch screen during the phone calls, keep reading, this might be the fix you need.
When I first got my iPhone 4, I was very excited. Put on the vinyl stickers, bumper frame and screen guard. All ready set go!!
Then problem happened. Whenever I answered the call, my face/ear touched the screen and activated all sorts of things, face time, 3 way calls, you name it, even hang up on people some time.
I thought damn, did I just get a lemon???
After a lot of soul searching, I came across couple of video demos on YouTube about the proximity sensor on iPhone 4. And yes I found the answer to my problem.
the guilty one - - - - screen guard!!!!! yes, you read it right, it is the screen guard that causes the problem. Let me explain a bit further.
In iPhone 3, you can use the Power button to lock the screen during phone calls. However in iPhone 4, Apple got rid of that function for the Power button. Now to lock your screen during phone call is purly performed by the proximity sensor. The sensor is hidden behind the screen. It is located about 4mm above the speaker slot on top of the screen. It measures roughly 1x6mm.
The tricky part is, it is totally invisible in a black iPhone 4. So you see only one slot on top of the screen. However if you google a picture of a white iPhone 4, you will see there are 2 slots on top of the screen. The top one is the hidden sensor.
I heard manufacturers have now altered their design in screen guard to include an extra slot to expose the sensor, however, almost all screen guards I come across on eBay, including the ones I used to sell, have only 1 slot. So when you put the guard on, the plastic shield works as a barrier to block the sensor causing malfunctioning.
A simple test to identify the problem.
make a phone call to your landline from your iPhone 4. Make sure it is NOT in SPEAKER mode, the screen should be brightly lit, while the line is open, lay the phone flat, slowly lower your hand to cover the top half of the phone. If the sensor isn't working, no matter how low your hand goes, even when it touches the screen, the screen is still active. Hence when you on the phone, you face/ear can activate the apps on the screen.
now, peel back the screen guard, do the same test again. I'll bet when your hand is approx 5mm above the screen, the screen will suddenly turn black (being locked by the sensor)
If this is the case for you, simply peel off the guard, cut a slot where the sensor is. Then put the guard back. As soon as the sensor is properly exposed, problem solved.
Hope this help!