iPhone 4S and 5 freezes after not answering 2 calls
After spending quite sometime trying to find out a definite solution to this annoying problem I am afraid this is a glitch we have to live with until apple comes up with a fix. In my case not even using Siri for a call helped out. Once the call is over the phone freezes again. The only walk around I have found is asking Siri to open any application I have in the phone (I usually ask for the calendar) and Siri will notice that the phone is locked and will ask for the four digit code to unlocked. Just plug the code in and magic, you will gain access to the phone without having to reset.