Nokia Lumia 720 User Manual page 116

You can set your phone to ask for the lock code
that you define.
Keep the code secret and in a safe place,
separate from your phone.
If you forget the code and your phone is locked,
your phone will require service. Additional
charges may apply, and all the personal data on
your phone may be deleted.
For more info, contact the nearest care point for
your phone, or your phone dealer.
IMEI This is used to identify phones in the network.
You may also need to give the number to your
care point services.
To view your IMEI number, dial *#06#.
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