T-Mobile has released its quarterly earnings report for Q4 of 2015, and it's good news for the uncarrier. The company reported net profit of $297 million in the holiday quarter — more than double the $138 million from the previous earnings. It also added 2.1 million net new subscribers (bringing its total for the year up to 8.3 million), of which 1.3 million were branded postpaid. This is below the 1.5 million postpaid subscribers added by Verizon in the same quarter, but comfortably above the 501,000 added by Sprint. T-Mobile's churn rate of 1.46 percent (the rate at which it loses customers) was also above Sprint's (1.62 percent), but below Verizon's (1.14 precent) and AT&T's (1.02 percent).

The growth isn't much of a surprise considering T-Mobile's aggressive tactics over the past year or so. Last August, the carrier officially overtook Sprint to become America's third-biggest carrier in terms of total subscribers, and has since continued to launch new promotional offers such as the controversial Binge On deal. The company has also been working on upgrading its network, with a recent report showing the carrier just squeaking past Verizon to offer the fastest LTE downloads in the US, and achieving significant gains in coverage (but still behind AT&T and Verizon).

However, despite these recent gains, the question is how long T-Mobile can keep up this growth. The company says it added a total of 4.5 million branded postpaid subscribers in 2015, but its outlook for 2016 has a target of 2.4 million to 3.4 million additions. CEO John Legere, though, remains characteristically undaunted: "We set out to change this industry, we're well on our way and we won't stop."