An exemplary method includes a user authentication system detecting a touch input on a lock screen of a mobile device while the lock screen displays a notification of an incoming voice call, determining that the touch input satisfies an end-user-defined authentication key requirement that includes an end-user-defined setting specifying that a direction of a vector of the touch input across the lock screen will not be considered as part of the end-user-defined authentication key requirement, and facilitating, based on the determining that the touch input satisfies the end-user-defined authentication key requirement, a pick-up of the incoming voice call. Corresponding systems and methods are also described.