Found an iPhone 4, How Can I Unlock it

I found an iPhone 4, and I want to know if there's any way to unlock it. I wanna use it with my tmobile account. Any help!
The Carrier is AT&T.

Report it as lost give it back to the owner you would want yours back wouldn't u

They can track it down either by apple the company and thers even an app in the appstore that says find my iphone and if they don't track it down they can also call the company and tell them to lock it so i can't use it unless you know ther account