The proximity sensor (just above the Verizon check mark) should be turning off/locking the screen when it's up to your ear. To test the sensor, while in a call put your hand over that area. If the screen doesn't turn off, try some canned air (or carefully blow) around the sensor area to clear any possible dust accumulation. If that doesn't work, try powering off phone, pull battery for a few minutes to reset things.