Most models have the number printed on the back of the handset or if back door comes off, it can be found under the battery pack.
OR
For Smartphones the IMEI number found by going to the Telephone KeyPad.
1. Press * # 0 6 # on your keypad.
2.Press ‘Call’ or ‘Send’ as if making a call.
3. Your IMEI number will be retrieved and displayed on your screen.