i am shocked nobody has mentioned the cause! it is simple, dictation is the problem. whenever you use dictation, iphone won't lock properly. you can verify this by listen to the speaker port when your phone is locked, there is a hissing sound coming out of it, reproducible 100% on iphone 4, 4s, 5 and 5s. you can fix this temporarily by reboot your phone, but fortunately for vo users, you just need to turn vo off and on again to fix this. just tell siri "voiceover off", tap the "listen" button, then "voiceover on", you're all set. do this again whenever you use dictation again, till apple fixes it. so now go tell apple about your finding, thanks. p.s., siri won't cause the problem, only dictation does.