Unused data can roll over for 12 months, and T-Mobile has promised to give all qualifying customers 10 GB of data to start the program rolling.
T-Mobile calls the program "Data Stash" with the tagline: "It's your data; what you don't use, you don't lose." The program is available to customers who buy one of its "Simple Choice" plans with 4G LTE data. Eligible customers must also buy smartphone plans that already have at least 3 GB of data or tablet plans with at least 1 GB of data.
It's the latest in a series of "Uncarrier" moves from T-Mobile. The initiatives are aimed at eliminating "pain points" that wireless customers feel when buying connected gadgets and data plans -- and a pointed challenge to larger networks such as Verizon, AT&T and Sprint. The first Uncarrier move eliminated annual contracts on T-Mobile, which now only lets you pay for your device and a pay-as-you-go data plan. After that, T-Mobile introduced a number of novel initiatives, including radical international plans, family plans and more flexibility for upgrades. Tuesday's announcement is the eighth in its series.
In an interview with Yahoo Tech's David Pogue, Legere said he's out to fix a "stupid, broken, arrogant" industry that he said collects $1.5 billion in overage fees every year.
"I detest the industry they set up," said Legere, explaining why T-Mobile continues to go after industry conventions.
It also probably doesn't hurt that this sort of populist attitude from T-Mobile has increased the network's reach with consumers. T-Mobile has 53.5 million customers, and Legere said that the company has had 10 million net additions in the past year and half.
"We're growing significantly, quickly," he said.
That customer expansion has come along with a serious network expansion. Two years ago, T-Mobile had no LTE coverage, but now it boasts that its high-speed network covers 260 million people in the United States. Legere said that the company is focused on expanding its footprint, particularly in rural areas, but that is in some ways a problem affected by the availability of airwaves.
Legere also said that customers will get 10 GB free: "We'll put 10 GB data stash in the bank." He said he got the idea for Data Stash in part from customers on Twitter who have been asking for this kind of program.