unlocking Gravity 2

i have a gravity 2 samsung i tried to unlock it , but there is a screen displays "sim unavailable contact your service provider'' how can i solve this matter ?

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Samsung Master Reset:
CAUTION: A "master reset" changes the phone settings back to default. This process will erase all contacts, downloaded settings, and downloaded applications (ringtones, wallpapers, graphics, etc). This process cannot be undone.

Enter *2767*3855# from the standby screen. (You may need to remove the SIM before it will allow you to type the code in)