Every iPhone has a unique number called an IMEI (International Mobile Equipment Identity) number that identifies it from all other phones in existence. This number allows carriers to track calls and data usage from the phone back to the network.
There are a couple situations where knowing your IMEI may come in handy. If you intend to carrier unlock your phone, you will need to submit your IMEI to your carrier to be considered for unlocking.
If you plan on purchasing a used phone from a third-party, you will want to check that the previous owner fully paid the phone off and that the IMEI number is clear, meaning the phone hasn't been stolen or globally blacklisted.
The easiest method for retrieving your IMEI is by going to Settings -> General -> About and scrolling down until you locate the IMEI section. You may tap and hold on the number to select it and copy it.
This is the preferred method due to the ability to copy and paste the IMEI number. Copying and pasting your phone's IMEI will save you time and ensure the proper number was entered when checking a device's IMEI is clear.
Another way to find your IMEI is by opening the Phone app, tapping Keypad at the bottom, and entering the following code: *#06#
The IMEI will appear on the screen. This method, however, will not allow you to tap and hold to copy and paste the IMEI. The number will need to be written down manually.
On the iPhone 6 and earlier models, you can locate your phone's IMEI on the back of your phone. It is listed at the bottom in very small print. If you are unable to read it, try using a magnifying glass or try taking a picture of it with another device and zoom in on the text.
As of the iPhone 6s and later, Apple has stopped etching the number into the back of the iPhone. But these newer models do have the IMEI etched on another spot.
The iPhone 6s and later have the IMEI number etched on the bottom of the SIM card tray. Simply remove the SIM card tray, flip the tray over, and read the 15-digit IMEI number listed under "IMEI".
This method, like the previous two, requires you to write/type the IMEI by hand.
In the event that you don't have your iPhone, you can locate the IMEI number on the original retail box that your phone came in. The number will be listed near the bottom next to the barcode.