The International Mobile Equipment Identity or IMEI number is a 15 digit unique number and every iPhone have one. The IMEI number is used by a GSM network to identify valid devices and therefore can be used for stopping a stolen phone from accessing that network. For example, if a mobile phone is stolen, the owner can call his or her network provider and instruct them to “blacklist” the iPhone using its IMEI number. This renders the phone useless on that network and sometimes other networks too, whether or not the phone’s SIM is changed.
How to find IMEI number on iPhone
The first way to find your iPhone’s IMEI is to enter *#06# on dialpad (just like making a call)
The second way is to access your iPhone’s Settings – General – About page
The third way is to connect your iPhone to iTunes