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Question: Limited Service Problem

Hi, I've got an unlocked G600, I've tried to put an O2 payg sim into it, but constantly get the message that there's limited service, so can't call or text. I've tried to set the network but it says that O2 is unavailable. Have another G600 on O2 that has no problems at all, so v. doubtful it's a network problem. It's driving me nuts! Any suggestions?