If you are pressing buttons with your cheek while using the phone to your ear, it sounds like you are experiencing problems with the proximity sensor. That is what blanks the phone's screen when holding it to your ear to prevent these things from happening.
First, do you have a case or screen protector on the phone? If so, try removing the case and testing. You should be able to look out of the corner of your eye to see if the screen is blanking. Is anything obstructing the upper portion of the phone where the front facing camera is? That is where the proximity sensor is. If any of that area is blocked, it can affect the sensor's performance. Also, make sure that you are pressing the phone to your ear completely. If you angle it away slightly, it can be enough to affect that. If that does not seem to change it. Try powering your phone off and back on again and see if that helps. If not, then make a Genius Bar appointment at the nearest Apple Store to have the device examined by Apple to ensure the sensor is working correctly or not.