Every phone has an identification code called IMEI number. This number is unique to each phone and your mobile network uses this to identify every phone connected to it. This number is very important in case you lose your phone. You will have to provide it to your mobile network provider to block any one from using your phone to make calls.
Finding out the IMEI number of your phone is pretty simple. Just dial *#06# and it will get shown to you. In case you have a dual SIM phone, you will get two numbers for each SIM slot.
Retrieving IMEI of a lost phone.
But 99% of us don't really care to write down the IMEI number of our phones for future reference. But if you are using an android phone, you could easily get back your phone's IMEI number any time. Just go to your Google dashboard page - https://www.google.com/settings/dashboard and expand the android tab. You will see a list of every phone from which you have signed into Google and their IMEI numbers as well.
How it works
While you sign in to play store in any phone, the login information including phone details are automatically uploaded to your Google account. This includes the phone IMEI number as well. And this is the information that Google makes available in your dashboard. So you will find details of all phones you have ever used to login to google from here.