If your mobile phone has lost then you can recover your mobile by IMEI number. For know IMEI number of your mobile dial *#06# . It will show IMEI number of your mobile. You should note this number to diary for further. This number will help you if your mobile has lost due to any reason.
But if you did not save IMEI number to anywhere and your mobile has been lost then there is another way to find out IMEI number of your lost mobile. Search the receipt and other original documents of purchased mobile. There is IMEI number on the documents and receipt of your mobile. You can find the IMEI number from those documents.