Source: T-Mobile US.

When scrappy wireless underdog T-Mobile US (TMUS 0.57%) presents another one of its "Uncarrier" strategies, cages rattle all around America's wireless communications market. That happened again last week, as T-Mobile introduced its seventh Uncarrier policy. Over the next two days following, T-Mobile shares rose 1,1% while AT&T (T 1.88%) and Verizon Communications (VZ -0.53%) fell by about 0.6% each.

T-Mobile CEO John Legere took the stage at an unveiling event in San Francisco, tied to a live audio webcast. "This time it's personal," roared the tag line for Legere's presentation.

The first 6 Uncarrier announcements mostly focused on lowering subscriber bills. Starting off by eliminating two-year contracts, T-Mobile introduced the first early upgrade in the industry (which now is a standard practice), removed roaming fees, and offered to pay off early termination fees when a new customer left another carrier a bit early. The latest installment separated popular music streaming services from T-Mobile's bandwidth caps, and gave customers a chance to "test drive" Apple 's (NASDAQ: AAPL) iPhone 5S in a free seven-day trial.

Uncarrier 7 is a bit different. Rather than reducing costs, this one's all about improving coverage. To do so, T-Mobile is leaning heavily on enhanced Wi-Fi calling features.

Wi-Fi Un-leashed
First and foremost, T-Mobile is rolling out Wi-Fi calling to every user under the popular Simple Choice calling plan. The company has actually offered free Wi-Fi calling features since 2011. But Wi-Fi calling only worked on a select handful of Android phones; Apple added support for this technology in testing versions of iOS 8, which will roll out to many modern iPhone models alongside the release of iPhone 6 this week.

The new version, dubbed Wi-Fi Un-leashed, will apply to the all of T-Mobile's new smartphone models and many existing ones, including updated or just-released iPhones. Moreover, the phone will be able to hand over connections between Wi-Fi and 4G LTE connections without dropping the call.

And, this version will offer HD Voice audio quality over both LTE and Wi-Fi links. That's another quality-boosting feature that T-Mobile presented early, and that Verizon only started offering last month.

"Wi-Fi Un-leashed is a game changer," Legere said in a prepared statement. "This is like adding millions of towers to our network in a single day."

How unique is this Wi-Fi calling feature?
Verizon doesn't offer Wi-Fi calling yet and AT&T won't offer this feature until 2015. So T-Mobile is smart to make a big promotional push behind Wi-Fi calling while the big boys are sitting on their hands.

Included in this Uncarrier launch, T-Mobile will open a sign-up window for its JUMP! upgrade program, for existing customers who don't have a Wi-Fi calling compatible phone yet.

Furthermore, T-Mobile will send out a Wi-Fi router to interested customers, free except for a refundable $25 deposit.

This device connects to your existing broadband service, then provides a Wi-Fi signal tuned for optimal voice-over-WiFi performance.

"It's like a T-Mobile tower in your home," explained T-Mobile spokeswoman Lindsay Morio in an email exchange. "At home, the Personal CellSpot prioritizes T-Mobile voice traffic over other types of Wi-Fi traffic (such as your kids are streaming a movie or our co-worker is watching YouTube videos) for crystal-clear HD Voice calls."

Pulling out all the stops. Source: Wikimedia Commons.

Will unlimited Wi-Fi calling help T-Mobile steal subscribers from the other major networks? Only time will tell, but as a Magenta subscriber myself, I'm happy to see a reduced load on its often overburdened network.

What can T-Mobile's new ideas do for investors?
What T-Mobile is doing here is actually quite simple. The underdog is putting pressure on the big boys -- AT&T and Verizon -- to either innovate or lose customers.

In this case, the Uncarrier announcement also aims to take some load off of T-Mobile's wireless networks by handing many calls over to hardwired broadband connections. So it's a two-pronged attack, improving the customer experience while lowering T-Mobile's infrastructure upgrade needs.

Verizon and AT&T are almost certain to follow suit eventually. That's usually what happens when a smaller carrier introduces consumer-friendly innovations that undermine the wireless industry's status quo. For example, minor-league player Alltel was first to market with voice-to-text messaging and "family circle" calling plans. Both of these became industry standards in short order, before being replaced by personal voice assistants and unlimited-everything plans.

But there's a window in between underdog introductions and industrywide adoption, where the innovator has a chance to snag subscribers from the slower-moving incumbents.

And it shows in T-Mobile's quarterly reports, as well as in its surging share prices.

In the four quarters between June 2013 and June 2014, T-Mobile gained 6.5 million wireless subscribers. That's a 15% year-over-year jump. Meanwhile, Verizon gained 4% or 5.3 million customers, and AT&T added 8.7 million accounts for an 8% jump. T-Mobile is playing in the same league as the big carriers now, and growing much faster on a percentage basis.

And in terms of investor returns, T-Mobile has scored a 22% gain since the summer of 2013 while both AT&T and Verizon showed negative returns. Investors appreciate the scrappy underdog, as long as its feisty tactics keep the subscriber additions coming.