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.