IMEI number can be found in the phones back under the battery mostly on white stick. In some cases there is written IMEI: <IMEI number> but in other cases there is only number written and it don’t tell you that it is IMEI number. IMEI number can be also found on the box package of the phone.
This is an example of the IMEI label on the phones back (under the battery):
Nokia 6020 IMEI label
Other option is even better. You can get IMEI number on screen without removing battery. You just need to type *#06# on your screen and it will show you IMEI number of your phone.
IMEI number in phone
IMEI number displayed
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