Follow This Steps :-
1) Download Privacy Lock from Here.
2)Create your pattern lock.
3)Select the apps which you want to lock.
4)Enable all permission for application.
5)Now select the App Cover -> Unknown Caller
6)Now Click On Finish
7) You Are All Set !
8) Now when someone wants to open any app which you locked then unknown call will come & Then if He answering the call or rejecting a call then also the application will not open at all.