But you do fully own the phone, even if you're on a contract. It's not like a mortgage or a car loan; the phone company doesn't have a lien on your phone. The penalty for breaking the contract isn't to give the phone back--it's to pay a termination fee.
This, specifically.
If they want to deal with ownership do some kind of leasing agreement... otherwise, let's not have public enforcement propping up the business model.