[Q] How to Unlock Sim after unlocking BootLoader?

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http://forum.xda-developers.com – Hi, today I successfully unlocked the bootloader on my xperia x8 via flashtool put on custom kernel nAa 14 and miniCM7 Yay Everything is fine apart that I want to change network because they have much better deals etc. I went to the 3 network shop and tried to put in the Sim, it took it in loaded but wouldnt register with the network , in fact I tried last week when I was on stock kernel and gingerxperia and it asked for ''unlock code'', today it didn't ask for unlock code took it in but wont register. I guess there must be some sim lock or something. I kindly ask you if you could give me a (HowTos)