You need to have 2 checks before you initiate a call.
1 - Accelerometer
2 - Proximity
3 - Initiate call

If you want to call someone or receive a call, you'll bring the phone towarts your ear. You can "calculate" this "move" with the accelerometer.
If the accelerometer picks up the gesture (that you put the phone to your ear), it'll activate the proximity sensor. If the proximity sensor is covered with your head/ear, it will initiate the call.