Activation requires your cellular carrier. Here's how to contact them:
Contact a mobile carrier
I have this same problem, I am not currently in the U.S, so I'm guessing I can not use it until I return? I was thinking it may have been a no service problem.
Are you running a Beta version of iOS7?
The Beta version has expired, and must be updated to the Public Release version of iOS7.
You will need to update by connecting to a computer running iTunes.
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