Hide my number in cellphone not working ...its sim or phone option?

i have tried a option in my mobile (samsung corby) hide my number....it was working previosly but now its not working i donno wats wrong ???
i checked the settings and its marked in Hide.......
is the hide option depends on sim or phone ?? anyways can i enable this option ??
pls get some idea .....
Update: hiya...Thanz if its phone ,,,y its not working all of a sudden ...it has a option 1. by network2. send 3. hide and its marked in hide even after saving the option again its showing my number??
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