Hello. It's an unfortunate event when a phone is lost or stolen, but here are a few thoughts:
1.) When you buy a phone, no matter if it has a SIM card or not, the IMEI is registered through your carrier. It's best to immediately notify your carrier that you suspect your phone has been lost or stolen. Your account can be suspended, and the phone can also be placed on a Lost/Stolen list, just in case that phone may ever be activated by someone else again.
2.) It seems to be overlooked when someone buys a phone, but insurance is almost necessary. It not only can have your back when it comes to breaking a phone that has glass on the front and the back, but also in times when you've lost the phone or it has been reported stolen. Insurance companies also place that phone on their own list of Lost/Stolen devices, just so that no one can use that phone again.
Find My iPhone is great if you've realized within about 30 minutes that your phone has disappeared - other than that, any half-educated teenager can figure out how to factory wipe the phone, leaving it to be forever lost in the world of Find My iPhone. At least if you flag it as stolen, the kid has to go and buy their own iPhone like everyone else.