You can use the IMEI (International Mobile Equipment Identity) to find a lost phone. However, you’ll need to know the IMEI (this is on the phone but also on its packaging, so if you still have the packaging but not the phone, you should be able to find the IMEI). Or, you can use Android Device Manager for Android phones or iTunes for Apple phones, to find the IMEI. Then, contact the police and your phone network with both your phone number, account ownership proof and the IMEI information and they’ll be able to track the phone if they're so inclined. There are also apps that allow you to track the IMEI too but you’ll obviously need a device other than your lost phone to operate any such app.