Full bars and can't make calls??

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http://forums.androidcentral.com – Currently, I'm in a place with no 4G or wifi signal at all. Once I get to the spot where I have 4 bars and try to make a call, a dialog box pops up and says "Emergency calls only". Once my signal comes back, why does this pop up?? Is there some kind of 4G signal booster that can be attached to the phone like a small antenna? (HowTos)