Source: T-Mobile

Last year was a good one for T-Mobile (TMUS 0.92%). It surpassed Sprint (S) to become the nation's third-largest wireless carrier. Its LTE network expanded to 305 million POPs and doubled in area covered -- nearly on par with AT&T (T 1.10%) and Verizon (VZ 0.90%). And it took 108% of postpaid phone subscriber growth in the industry.

While T-Mobile added 3.5 million net new phone subscribers, AT&T, Verizon, and Sprint combined to lose 260,000. AT&T lost about 1.7 million postpaid phone subscribers, Verizon added a bit less than 1.1 million, and Sprint added 390,000.

After beating down AT&T last year, T-Mobile appears to have turned its attention to Verizon. Can the Un-carrier keep taking share of the market in 2016?

A look at porting trends
A good metric used to see where net additions are coming from is the porting ratio. When a customer switches carriers, he "ports" his number to the new carrier. The porting ratio measures how many customers joined a carrier (T-Mobile in this case) versus how many left. A porting ratio higher than 1 indicates the carrier is adding subscribers from a competitor, while a porting ratio less than 1 indicates a net loss to the competitor.

T-Mobile doesn't discriminate when it comes to taking subscribers. Last quarter its porting ratio for AT&T came in at 1.92, for Verizon it was 1.44, and Sprint came in at 1.56.

T-Mobile's porting ratio with Sprint is down from the third quarter, when T-Mobile added 2.09 subscribers for every customer it lost to Sprint. Sprint added T-Mobile to its "cut your bill in half" program last quarter, and it seems to have drawn some subscribers away. But trends are returning back to normal, as CEO John Legere noted that the porting ratio climbed back to 1.8 in the week before its fourth-quarter earnings report. Considering Sprint's subscriber base is much smaller than AT&T and Verizon and T-Mobile's porting ratio is still very strong, the temporary drop isn't a big concern.

The trend is not looking good for Verizon. T-Mobile's porting ratio with the leading carrier climbed to 1.44 from 1.33 in the third quarter. The week before T-Mobile's earnings release, it climbed to 1.5. It seems T-Mobile's constant attacks on Verizon are started to pay off, including its Super Bowl ad featuring Steve Harvey and its "BallBuster" commercial.

AT&T continues to lose customers, and T-Mobile's porting ratio with the company remains high, floating around 2:1. AT&T is going after high-value customers looking for the quadruple play with its recently acquired DirecTV. Customers seeking value or just a phone service subscription are looking elsewhere to connect their devices.

What's behind those numbers?
While the trends are favoring T-Mobile, it's important to understand what's driving those numbers, and if they're sustainable.

By far the greatest factor are the improvements to its network. Its LTE network now covers 305 million people. Additionally, its low-band 700 MHz spectrum -- which improves coverage inside buildings -- covers 190 million people, with additional deployment scheduled for 2016.

The improvements to T-Mobile's network has reduced churn at the carrier -- i.e., the percentage of subscribers that switch. Last quarter T-Mobile's churn rate fell 27 basis points year over year to 1.46%. On the earnings call, management forecast that churn in the first quarter will set a new record low.

Another big factor are T-Mobile's Un-carrier promotions like Binge On and Music Freedom, which allow subscribers to stream video and music without having it count against their data caps. With regard to those promotions, T-Mobile CEO John Legere told investors, "These qualitative items along with the commitment that we will continue to add to this is a huge differentiation in our brand and why the competitive environment continues to play in our favor." So customers can expect even more value in the future.

As T-Mobile deploys its 700 MHz spectrum and rolls out more Un-carrier events, the carrier will only become more attractive to subscribers of both T-Mobile and its competitors. With fewer customers leaving and more customers joining, the Un-carrier is poised to add even more subscribers this year.

Management guided for total postpaid net adds of 2.4 million to 3.4 million, well below its 8.3 million total postpaid net adds last year. That estimate may prove to be rather conservative.