Please elabourate so one of us can assist your situation.
What do you mean by Security code? The one on your Account (BAN) that the CSR asks you to verify your identity? the one your phone asks when you pop in a new SIM card or power up your phone? or some other code being asked? like PUK or something else?
The one that your phone asks you, is documented in your Phone's Owners Manual, unless you have changed it to something else, then it would be something you picked.
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