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The carrier grew 8% relative to its total share of subscribers in the wireless market, the report says.
"In the mobile world, 8% is enormous," said Michael Levin, one of the authors of CIRP report said in an interview with Business Insider.
The results come from interviews with 500 people who activated a new phone during the fourth quarter of 2013. In the report, CIRP says T-Mobile's adoption of the iPhone, contract-free service plans, and smartphone leasing plans all contributed to the carrier's gains that quarter.
Out of all the smartphone activations during the period, AT&T accounted for the most with 29%. The CIRP report also says that Sprint was the only carrier that lost customers during the period.
To be clear, T-Mobile is still much smaller than AT&T,