People tend to lost their iPhone far too easily in restaurant, bar and public places. You might by chance found a lost iPhone. If the phone is unlocked, you can look through the address book, recent call list and call a family member or friend to let them know where the lost iPhone is.
You have no access to the contact list if the iPhone is locked. You can try if Siri can help. With the locked screen on, activate Siri by holding the iPhone up to your ears. Try “call home”, “call my mom”, “call my dad” to see if Siri can connect you to a family member.
Please note that Siri might not come out on a locked screen as this default behaviour can be disabled from Settings.
If the lost iPhone is setup with Find My Phone, the owner has the ability to track down the location of the iPhone or flash a contact number on the locked screen for you to call. Just make sure you do not turn off the phone.