is there even a sim card inside it?
very likely, the phone was already listed as stolen, and replaced. Therefore the sim is inactive.
If you had another gsm phone, then you could get to the contacts.
If you really want to return it, just take it to the police.
Disclaimer: If this is an answer to a TECHNICAL QUESTION, then MY ANSWER is correct, even if its contradicting something that a tech support rep, or anyone else said. I am not giving you the answer YOU WANT, I'll give you the answer I know is CORRECT.
If this is about CARRIER POLICY, then you should probably be aware that carriers change their policy unexpectedly, and inexplicably. Even if the answer is correct when posted, it could very well be wrong in a day, or week.