iOS 6 and above freezes on iPhone 4 and 4s randomly.
Step 1. Your phone should have Assistive Touch enabled.
Step 2. When the phone freezes, tap on Assistive Touch and click on Voice Control.
Step 3. Speak a name to dial from your address book and wait until the dialer pad appears and cut the call by pressing the end button.
Step 4. That's it...your iPhone is back in action. Slide open and unlock.