Here's the motive behind T-Mobile's big Wi-Fi push

Wi-Fi is the latest weapon T-Mobile is using in its battle for wireless customers: Beginning Sept. 17, T-Mobile customers will be able to make and receive voice calls and messages over Wi-Fi.

The advance here has less to do with cost than with coverage; unlimited calling and texting has become standard fare in wireless plans. Instead, this is more about coverage and call reliability.

A customer exits a T-Mobile store in Glendale, Calif.
Robyn Beck | AFP | Getty Images
A customer exits a T-Mobile store in Glendale, Calif.

By allowing customers to field calls using their cellular number over Wi-Fi connections, T-Mobile should be able to both ensure better call quality and free up bandwidth on its cellular network for new subscribers.

"What we're announcing today is that every single device that we sell in the store, every smartphone, will come out of the box Wi-Fi enabled, ready to go," T-Mobile CEO John Legere told CNBC. "And we're gonna offer a one-time, special upgrade period where people who don't have Wi-Fi-capable smartphones can come in and get that. And what happens now is every Wi-Fi hotspot has become a T-Mobile tower."

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T-Mobile isn't the first with this kind of technology—AT&T has offered its MicroCell for years, which is different technology that does effectively the same thing. What's new is that T-Mobile is both allowing Wi-Fi calling over any Wi-Fi network, and offering a Wi-Fi router for a mere $25 deposit.

T-Mobile announces Wi-Fi calling & texting
T-Mobile announces Wi-Fi calling & texting   

As part of the initiative, T-Mobile also is encouraging subscribers to upgrade to one of the newer phones in its lineup, all of which now support Wi-Fi calling.

The Wi-Fi push isn't limited to on-the-ground messaging, either. T-Mobile also announced that it has teamed up with Gogo to offer free inflight messaging to subscribers who have enabled Wi-Fi calling.

Those customers will be able to send and receive text and picture messages, and check voicemail, for free on Gogo-equipped flights.

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T-Mobile is trying to exploit the hoopla surrounding the Sept. 19 release of Apple's iPhone 6 with its new twist on Wi-Fi connections.

The iPhone 6's latest options include the ability to begin a call on a Wi-Fi network and then automatically transfer the conversation to a cellular network without interruption when the device is on the move. T-Mobile is the only U.S. carrier offering this versatility on the iPhone 6.

—By CNBC's Jon Fortt. The Associated Press contributed to this report.