The backup pin is the number you have set yourself. There is no default PIN that you can use to unlock the phone. If you've set the PIN and forgot it, then the only way to access the phone is through the Factory Reset method that wipes the whole phone (custom, settings, wallpaper, apps, etc.). To access Recovery menu, press Volume Up + Home + Power button. After the Android logo appears, release the buttons and select Wipe Data/ Factory reset. Reboot the phone and that's it. Next time you set a PIN, make sure to remember it or write it down on a note.