There is no doubt that T-Mobile has had an incredible run of success over the last year and a half. However, there’s some real question about who T-Mobile’s success has been hurting the most — for while the “Uncarrier” loves to portray itself as the scourge of Verizon and AT&T, those two mega-carriers have not been losing subscribers or hemorrhaging money. Instead, it seems that T-Mobile’s gains have come at the expense of smaller prepaid wireless carriers.

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Consumer Intelligence Research Partners just shared some new data with us showing that both Sprint and T-Mobile made impressive gains over the past quarter by increasing their total shares of phone activations. However, both AT&T and Verizon also showed gains in total share on the quarter, albeit much smaller gains than the ones T-Mobile and Sprint posted. Instead, the biggest drop in total activation share came from the “other” category that’s reserved for small-time carriers.

Source: Consumer Intelligence Research Partners

CIRP says that despite the onslaught of new deals being offered by the two underdog carriers, AT&T and Verizon customers have remained very loyal so far.

“AT&T and Verizon continue to have the most loyal customers,” says CIRP cofounder Josh Lowitz. “Relative to their size neither attracts as high a percentage of new customers from other carriers as Sprint and T-Mobile do. Sprint and T-Mobile also attract a somewhat higher percentage of first-time phone buyers, as we would expect from carriers that offer lower-cost plans.”

One likely reason AT&T and Verizon users are loyal is because their two networks offer broader coverage than the networks of Sprint and T-Mobile. This is why next year’s planned auction of spectrum on the 600MHz band is so crucial for America’s two wireless underdogs: It’s very hard to see how they can really compete for the higher-end AT&T and Verizon customers if they don’t significantly improve their coverage.