I jailbroke my Iphone and Cydia is installed but I can't use my T mobile SIM card what do I do?

When the ATT SIM is in it says no service and when the T mobile SIM card is in it's little bar but nothing goes through I thought jailbreaking also unlocked it. So what do I do?
Answers:
If it is a 2G, the Redsn0w jailbreak will unlock it at that time. If it is a 3G or 3GS, you have to jailbreak it and after in cydia, you have to install Ultrasn0w to unlock it.


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