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.

Prior to joining BGR as News Editor, Brad Reed spent five years covering the wireless industry for Network World. His first smartphone was a BlackBerry but he has since become a loyal Android user.