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Some phones have the option to allow calls and texts from CONTACTS only.

To see if your phone will do this, go to Menu>>Settings and Tools >> Phone Settings >> Security >> Restrictions >> Messages >> from there you can choose contacts only...etc.

The lock code (if you have not set one yourself) is the last 4 digits of your phone number. Your menu choices may be different, but I hope I've given enough information to lead you in the right direction.