No service after jailbreak/unlock 3GS 3.1.2

jailbreak is not unlocking it. you need to get ultrasn0w via cydia.

how to get ultrasn0w

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• Ensure that you have upgraded to iPhone OS 3.0
• Jailbreak your iPhone 3G using redsn0w or PwnageTool (this will also install Cydia/Icy)
• Run Cydia or Icy
• Add the repo repo666.ultrasn0w.com to Cydia or Icy. That last “o” is actually the number zero “0”! If you use the letter “o” you’ll get an error.
• Search for ‘ultrasn0w’ in cydia or icy and install ultrasn0w
• Reboot your iPhone 3G
• T-Mobile USA users should disable 3G before using ultrasn0w. To disable 3G, go to Settings->General->Network on your iPhone and turn off 3G Mode.

Read more: http://techblissonline.com/download-ultrasn0w-and-unlock-iphone-3g-os-30/#ixzz0tcWcEKLD

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http://techblissonline.com/download-ultrasn0w-and-unlock-iphone-3g-os-30/