It shouldn't be a big surprise that T-Mobile (NASDAQ:TMUS) is still winning over more customers from its competitors than it's losing. But CFO Braxton Carter gave some details about how much better T-Mobile is doing in the fourth quarter versus Verizon (NYSE:VZ), AT&T (NYSE:T), and Sprint (NYSE:S) than it did last quarter.

"So far, in the fourth quarter, our porting ratios are up significantly," Carter said at a recent investors conference. "We ended the third quarter at a porting ratio of 1.53. We're now 1.73 on porting ratios to date this quarter. And our porting ratios are up against every carrier."

So, what exactly continues to drive higher porting ratios for T-Mobile -- the ratio of customers moving their phone number to T-Mobile over the ratio moving their number away? Carter pointed to two key factors: the new iPhone release and its decision to bundle Netflix (NASDAQ:NFLX) with its unlimited plans.

T-Mobile Store in Times Square, New York

Image source: T-Mobile.

T-Mobile has one of the smallest iPhone bases of all the carriers

T-Mobile was the last of the four major carriers to start offering the iPhone to its customers. As a result, its base of iPhone users is relatively small, especially compared to AT&T and Verizon. So, whenever a new iPhone comes out, it's a big opportunity for T-Mobile to win over new subscribers as they upgrade their phones.

In the past, T-Mobile has been ultra aggressive in offering discounts on the iPhone. But its offer was much more moderate this year after it found its competitors copying its promotions, practically nullifying any positive impact.

Carter views the higher price of the iPhones, particularly the iPhone X, as a factor that could cause consumers to really evaluate their carrier options. Considering T-Mobile has positioned itself to provide one of the best values in wireless service, it's well positioned to capitalize on consumers looking to offset higher device prices.

Around two-thirds of all iPhones in the United States are models from more than two years ago. Nearly all of those devices should be paid off or close to it if they were on equipment installment plans. That represents millions of potential customers. If just a few extra thousand switch to T-Mobile when making their decision, it could have a big impact on the company's porting ratios.

Pushing its way into bundling

Unlike AT&T and Verizon, T-Mobile is a stand-alone wireless carrier. It doesn't have any services it can bundle.

Since acquiring DirecTV, AT&T has been able to offer a nationwide service bundle for its wireless and television business. This year, it's taken bundling a step further, offering free HBO to its wireless subscribers and offering discounts for its various television packages. Verizon also has a small wireline business it can bundle with wireless in the regions where it operates video and home internet service.

T-Mobile responded in September by offering free Netflix to its subscribers. Netflix has over 51 million paid subscribers in the United States as of the end of the third quarter. It's very likely the majority of households are paying for Netflix. So, T-Mobile isn't pushing something its customers probably don't want to boost revenue; it's picking up the tab for something customers are already paying for out of pocket.

Bundling Netflix is producing better user retention for T-Mobile, according to Carter. There's now a switching cost for customers thinking about leaving T-Mobile, just as there is if a customer wants to ditch AT&T's bundle of wireless, TV, and HBO.

Sprint recently copied the strategy, offering its customers free Hulu. Hulu doesn't have quite the same demand as Netflix, so it likely won't have as strong of an effect as offering Netflix does.

T-Mobile is off to a strong start this quarter. And as we head into seasonally strong periods for both the iPhone and Netflix, it could get stronger as the quarter progresses.

Adam Levy owns shares of Verizon Communications. The Motley Fool owns shares of and recommends Netflix and Verizon Communications. The Motley Fool recommends T-Mobile US. The Motley Fool has a disclosure policy.