Don't worry about broken Sim card...While Unlocking You no need Sim card,so after Unlock it You can buy new sim card,So you need to specify the NETWORK and Country it locked now.Get the Unlock code from here
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Mobileunlockguide.com and Unlock it step by step,Unlocking instructions:
Warning: Do not enter any wrong unlock unlock
codes on your Sony Ericsson C902 mobile phones. If you enter over a
certain amount of wrong unlock codes on your mobile phone, your phone
may get hard locked and sometimes become unusable.1 - Switch ON your Sony Ericsson C902
without any SIM Card,
2 - Press the
left arrow key/button
once,
3 - Press the
button * twice,
4 - Press the
left arrow key/button
once,
5 - The message "Personalize with MNC2" will appear, (or you will have
displayed an unlock menu, select the lock you want to unlock)
6 - Enter the Unlock Code.