How do I put a lock on my phone?

Okay so, I have the enV touch and my mom found it at her work and nobody claimed it so she gave it to me, And I don't knw the last for digets of the phone from before. So when I go to put a lock it says" enter the last 4 digets of your phone number" But I don't know the last persons phonw number. How could i do a lock?