An IMEI (International Mobile Equipment Identity) number is a unique code that identifies your phone. Phones with two SIMs have two IMEI numbers, one for each SIM. This allows the phone to be identified separately on each network that the SIMs connect to.
You'll need your IMEI number if you contact support regarding a warranty and it's important to have if your phone is stolen. Service providers can use the IMEI to block a stolen phone from mobile networks and many police departments keep a record of stolen phones using this number.
You might never need your IMEI number, but it's a good idea to record it, just in case.
To find the IMEI number:
You can also find your IMEI number by dialling *#06# in the Phone app. On some phones, the MEID HEX number is your IMEI number.
If your phone doesn't turn on, check the box that it came in or the receipt for the IMEI number.