During its fourth-quarter earnings call, Comcast's (NASDAQ:CMCSA) management disclosed Xfinity Mobile, its wireless phone service, reached 380,000 subscribers. That's up 187,000 from the end of the third quarter. The service rolled out to select markets in May and expanded to Comcast's full footprint in August.

Still, Comcast's 380,000 subscribers through six months is well below the net phone subscriber additions of Verizon (NYSE:VZ) and T-Mobile (NASDAQ:TMUS). The two carriers added 431,000 and 891,000 phone subscribers, respectively, in the fourth quarter alone. While Comcast's early success is notable for the company, the lack of acceleration in subscriber growth during the seasonally strong fourth quarter for phone subscriber additions indicates the competitive threat doesn't amount to much so far.

Comcast says it plans to accelerate the business in 2018, though, and it could start to meaningfully impact carriers like Sprint (NYSE:S) and AT&T (NYSE:T) more than Verizon or T-Mobile.

IMAGE SOURCE: COMCAST.

Xfinity Mobile competes on price

Comcast offers Xfinity Mobile service exclusively to its home internet service subscribers. The wireless service relies on Comcast's network of Wi-Fi hotspots as well as Verizon's wireless network through an MVNO agreement.

Customers have two payment options: "by the gig" for $12 per GB of mobile data or a $45 per month unlimited plan. Those prices make it an extremely cost effective option for most wireless subscribers -- at least those that already subscribe to Comcast's home internet service.

That makes Xfinity Mobile a meaningful threat to carriers that compete more on price than their service -- specifically Sprint. It has been the most aggressive of the four large U.S. carriers in offering lower prices to its subscribers. It's now starting to change its tune after its planned merger with T-Mobile fell through. Sprint is looking to increase its pricing, which will make it harder for it to attract more customers and keep the ones it has -- especially if another low-cost competitor (Comcast) is servicing about half the country.

Bundling economics

Comcast is able to compete on price because it requires customers to also subscribe to its high-margin home internet business. In fact, the company is losing money on the wireless service, posting an EBITDA loss of $480 million last year. Management expects that number could increase by $200 million this year as it ramps up its customer acquisition efforts.

As such, Comcast's service represents a comparatively bigger threat to AT&T, which has recently begun relying on bundling economics itself to sell its services. Sprint and T-Mobile also bundle to a lesser degree by partnering with Hulu and Netflix, respectively. Verizon, notably, doesn't promote any bundles to its wireless subscribers.

AT&T is using bundling primarily to support its television business, which is even larger than Comcast's. It will also bundle home internet service for customers. Comcast is going head to head with AT&T's bundle, and it has a huge market to sell into. After all, almost 24 million residential homes subscribe to Comcast's high-speed internet.

Customers that aren't looking for a bundle are probably better off going with Verizon or T-Mobile.

Some carriers are more at risk than others

Comcast's lack of acceleration in subscriber additions in the fourth quarter after expanding to more markets is curious. Perhaps it indicates Comcast isn't making a big marketing push with Xfinity Mobile. Management expects customer acquisition costs to increase next year by a significant amount, which should result in a growing number of subscribers joining every quarter.

Still, Comcast is largely competing on price by using bundling economics. That's a bigger threat to Sprint and AT&T, which use those as their distinguishing factors, than it is to T-Mobile and Verizon, which have found other ways to differentiate their services.

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