Comcast (CMCSA 0.69%) made a couple of big changes to its Xfinity Mobile pricing this week. Customers with two or three lines on their unlimited plan will see their bill slashed to just $30 per line. Meanwhile, customers with four or more lines can add additional lines for just $20 each.

The new pricing is especially appealing for those on smaller family plans, and Comcast was keen to highlight the savings for those customers versus the big three wireless carriers -- namely, AT&T (T -1.19%), T-Mobile US (TMUS -0.26%), and Verizon Communications (VZ -2.39%). The new pricing may be a response to worries about the company's broadband business.

Comcast is leveraging mobile to save broadband

The head of Comcast's cable division, Dave Watson, told analysts during the company's second-quarter earnings call that he plans to continue the company's aggressive push into mobile. "What we won't do is chase pricing down to the bottom," he noted. Nonetheless, that's exactly what he's doing. The new price changes and the press release accompanying them are heavily focused on price, while briefly mentioning the value of Comcast's service.

The step up in competitiveness may be a response to worries about its broadband business. Subscribers remained flat in the second quarter, leading investors to wonder whether the profitable business segment is going to start suffering a setback.

Over the last couple of years, Comcast has been focusing on the bundle of broadband and wireless service instead of the bundle of video and broadband. In fact, it's mostly grown indifferent as to whether someone subscribes to video at this point, as judged by its pricing and push into streaming. But if it can get a broadband subscriber on its wireless plan, it substantially increases the likelihood that the subscriber will keep both services for a long time.

Indeed, that's the same approach T-Mobile is using, except attacking the market from the other side. With an established wireless customer base, T-Mobile is also looking to sell potential subscribers a home internet service that uses its wireless network. T-Mobile had the most net additions of any broadband internet service provider last quarter.

And while T-Mobile's home internet service is limited by its wireless network capacity, it's working to expand its network while offering limited home internet service in markets where it can't yet offer unlimited data.

Comcast is fighting back against the big three the only way it can

T-Mobile and Verizon are clear threats to Comcast with their fixed wireless access services. The two combined to account for more than 100% of the broadband industry's net additions in the second quarter (subscriber churn makes this possible). Meanwhile, AT&T is expanding its fiber network, which can offer high-speed connections to the home.

The best way for Comcast to defend its profitable home internet business against other telecom companies is to offer its subscribers the same services as its competitors but at a better price. With its profit center firmly in the broadband internet business, management can be more aggressive with its wireless service price. That's especially true because its wireless subscriber base is still relatively small.

So the overall impact of the price change on Comcast's bottom line is minimal, especially when you take into account the media business it also operates. At the same time, it's posing a meaningful threat to the big three wireless carriers, which are coming after its bread and butter.