Today, AT&T's profit margin in wireless may be even wider than those of its rivals, Hallaren said. But the carrier's results have probably been buoyed by its exclusive deal to sell the popular Apple iPhone, Hallaren warned. That arrangement is widely expected to end around the middle of this year.

"AT&T has so underspent that they're going to get hit with a double whammy" when iPhone exclusivity ends, Hallaren said.