To obtain the correct IMEI number type *#06# on the phone's dial screen as if you were going to make a call.
A unique 15-digit number will immediately appear on the screen. If you see a longer number, simply note down the first 15 digits without spaces or hyphens.
You may also check the IMEI number by clicking on the information icon “i” on the device's welcome screen.
Make sure you only use one of the methods described above to obtain the IMEI number of the iPhone. Do not trust the number printed either on the back of the phone, the box or on the sales receipt as it may not coincide with the iPhone's actual IMEI.