If you like the case your using, turn off "raise to speak" in the settings for siri and it will help. The problem is that the 4s proximity sensor was moved to left a little and it causes some cases that cover the sensor to make the phone think its next to your face. You can test this by activating Siri while not in its case and cover the top part of the phone and it will transfer the volume to the ear peace and will turn off the screen. Your case also will cause your screen to dim if you use auto brightness in the settings due to the covering of the sensor. I hope this helped.