Please dial *#06# on ur phone.The lengthy number that appears on what looks like a Barcode,is called ur phone's IMEI NUMBER.With this number,ur last/ current location can be traced even if the sim card had been removed from ur phone.
Give ur IMEI NUMBER to family and friends so that in the event of (God forbid) your disappearance without an explanation and ur phone is off or rings unanswered, they can make the case easy for the police by providing them with this number which can then be used to track ur movement.