On the SIM tray of iPhone 3G

On the back of your iPhone

Your iPhone’s serial number and IMEI are engraved on the back metal case as shown in the photo below:

iPhone About screen–Locate Serial Number, IMEI, and Modem Firmware

You can find your iPhone serial number, IMEI, and modem firmware in the About screen on your iPhone.
From the Home screen, tap Settings > General > About.

iPhone packaging

If you still have the original packaging that your iPhone came in, you can find your iPhone serial number and IMEI printed on the barcode label that’s affixed to the packaging.