Report the lost phone to the provider immediately. The operator could block the number and will stop anyone from using it. It would be best to report it to the authorities as well. Setting up a passcode can help slow down a thief from using the phone right away. The IMEI number can also be registered to the provider to tag the phone as stolen. Smartphones often have built-in security that should be used and as a final precaution, think about having mobile phone insurance.