I am using Nokia Lumia 620, Recently my phone unresponsive to calls when it in locked. (ie, while someone calling to my phone,it shows"slide up", but when I try to slide up,its not responding)
Just as teckris says it happened from time to time that my HTC 8X won't allow me to slide up the lock screen (it just starts to slide but then drops). A lock/Unlock of the phone solves the problem for me.