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As far as signal/usage issues, it ALL depends on where you live.

Exactly. Ask your neighbors/co-workers if they use ATT by you and what their experiences are.

Around Metro Detroit, ATT works ok, but not nearly as good as my Verizon service did :-/. No major issues with dropped calls or anything, mostly just a signal strength issue. I cannot recieve calls when I'm in the grocery store on ATT, but could fine on Verizon (and this is not limited to iPhone; my wife had ATT a few years back and had the same issue).
 
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