contact a number from the phone you found and tell the person that you have found this phone and ask them whos number it could be maybe they could tell you who it belongs to.
Look in the recent calls, choose the phone number that the person calls most,call that number, and expalin you found the phone, and want to return it to the owner,
If the phone still has a charge left to it, and you can turn it on you can do the following. Look through the contacts, and make a call to "Home, Mom, Dad, Wife ect", this will help you locate the owner.
