My iPhone 4 won't receive 3G?

I got my iphone 4 about a week ago and it won't get a 3G signal. It hasn't had one since i got it and i don't know if it is the phone's or O2's fault, i've tried resetting the network settings and still nothing has happened… I'm not sure what to do, what do you think the problem is?

Why don't you take it to O2.It might be a problem with the phone.

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