You can locate all important device-related ID numbers on the iPhone itself in the General section of Settings. You can also connect it to your computer and, in iTunes, view the Summary tab for the device. Both methods give you access to the iPhone's serial number, Mobile Equipment Identifier and Integrated Circuit Card ID.
Determining IDs Using the Settings App
To locate device-related IDs on iPhones running iOS 6 or later, tap the "Settings" icon on the home screen to launch the Settings app. Tap "General" on the main menu in the Settings app and select "About." Here you'll find several identifying numbers for your iPhone, including its serial number, Mobile Equipment Identifier and Integrated Circuit Card ID.
Determining IDs Using ITunes
Alternatively, connect the iPhone to your computer via its USB cable. Open iTunes on your computer if it doesn't launch automatically. Select your iPhone on the list of devices displayed on the horizontal menu bar. Click the "Summary" tab to view device information. Your iPhone's serial number is displayed next to the Serial Number heading. Click your phone number to view your Mobile Equipment Identifier and Integrated Circuit Card ID.