Encrypt your data
Want to protect your data against unauthorised use? You can encrypt data on your phone using an encryption key.
Select 
> Settings > Phone > Phone management > Security settings > Encryption .
It may take several minutes to encrypt or decrypt your data. During the encryption process, do not:
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Use your phone unless you have to
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Switch your phone off
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Remove the battery
If you haven't set your phone to lock itself automatically when not in use, you are prompted to do so when you encrypt your data for the first time.
Encrypt the phone memory
Select Encryption on.
Decrypt the phone memory
Select Encryption off.