Subsidies to get iPhones in more hands have a way of adding up.

AT&T's success in selling Apple's iPhone  is haunting the top U.S. phone carrier, at least for now. Because AT&T offers subsidies on the iPhone to help discount its price for consumers, strong sales of the gadget will boost the carrier's revenue but hurt its profit margins. That weighed on AT&T's third-quarter results and drove shares down 6 percent on Wednesday.

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