The Guardia Civil have issued advice that can help if a phone is lost or stolen.
The Guardia's advice is to obtain the serial number code - IMEI, International Mobile Equipment Identity. This can be done by typing in the short code *#06#; the number should appear on the screen.
A note of this serial number should be kept. The service provider should be informed, and it will block the phone.
There are other ways of obtaining this code, e.g. it can be found inside the device.