Let's say you don't use all your cell phone data this month. Normally, whatever is left over goes away, but if you're a T-Mobile customer, that unused data will now carry over to the next month.

The self-proclaimed "un-carrier" on Tuesday announced the latest in a string of moves intended to take on rivals and win over new customers. Going forward, any high-speed data you don't use in a given month will automatically roll into a personal "Data Stash," so you can use it at a later time, for up to a year.

Starting in January, Data Stash will be available at no extra charge for every new and existing T-Mobile customer on a Simple Choice plan who has purchased additional 4G LTE high-speed data for their device (3GB or more for smartphones and 1GB or more for tablets). Better yet, every Data Stash comes pre-loaded with 10GB of 4G LTE data for free (though your data will not start rolling over until that 10GB has been used up).

Any unused data will be rounded up to the nearest megabyte and automatically added to your stash at the end of the month. Then, when you need more data, you'll automatically start tapping into your data deposits.

T-Mobile is the first national wireless provider to implement this type of program. Typically, any leftover data a customer has already paid for is confiscated, T-Mobile said.

"Can you imagine your gas station siphoning unused gas from your car each month? The U.S. wireless industry is even worse," T-Mobile's outspoken CEO John Legere said in a statement. "Americans have been gamed by the carriers into buying huge data plans — all to avoid getting screwed with overage penalties. Only to find out they bought more than they need which is then confiscated by the carrier. For the consumer it's lose, lose."

There's no limit to how much data you can collect in your stash. The deal is available on individual, family, and business accounts.

Though T-Mobile has been introducing some innovative products lately, many mobile users have been reluctant to make the switch because of the carrier's coverage, which is good in big cities but gets spotty in more rural areas.

But T-Mobile has been working to expand its LTE network, which now covers 260 million Americans, an increase of 10 million people in the last 60 days alone, the company said. T-Mobile also today announced it has deployed its newly acquired 700MHz spectrum in Cleveland, Colorado Springs, Minneapolis, and Washington, D.C., extending coverage to less-populated areas and improving in-building service. In addition, Legere and Co. yesterday that its new Wideband LTE, which boosts network speeds up to 50 percent, is now live throughout the greater NYC metro area.

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