This sounds a bit complicated but it really isn’t.
The International Mobile Equipment Identity (IMEI) number is a 15 digit serial number unique to each and every phone. You’ll usually find it printed on the back of the handset or on the SIM gate of most Apple iPhones.
Alternatively you can ask your phone to tell you itself: it’ll display its IMEI on the screen if you enter: *#06#
A 15 digit number should be displayed.
Example: 359395000322140
If your handset displays more than 15 numbers or is separated by symbols,
please enter first 15 numbers only.
Example: 351965-00-145687-9-03 (Enter: 351965001456879 only)