Recently jailbroke Iphone 4 IOS 6.1.3 - My Sim card wont work in the phone - Help

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I recently purchased a jailbroken Iphone 4 - It was working fine then the device wouldn't start up so I put it in recovery mode and jailbroke it using Redsn0w. Everything seemed to work fine bar the fact I have no service. The phone was previously locked to T mobile UK and I am on Three in Ireland. Any help would be greatly appreciated as I am only new to the jailbreak software.

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