Can a stolen/found iPhone be activated?

My friend found an iPhone and i want to buy it. Its on the AT&T network. If I put in a different sim card will I be able to use it? Would it make a difference if I jail break it? Any information will help.

No because maybe the user, reported the phone stoles, or lost and At&t probably has all the information, or also the iphone user could locate the iphone fast.