With the new Un-carrier 8.0 program, T-Mobile becomes lets you roll your unused high-speed data into the next month. Starting in January 2015, Data Stash will be automatically available to every T-Mobile customer – every individual, family and business – with a postpaid Simple Choice plan who has purchased additional 4G LTE data, 3GB or more for smartphones and 1 GB or more for tablets.

In addition, T-Mobile announced they will start every Data Stash with 10 GB of 4G LTE data for free. That’s 10 GB of free data for every line in the family on a qualifying plan.

But T-Mobile isn't alone in offering data flexibility. Other carriers do it by letting families share a collective pool of data within a given month. T-Mobile does not offer data-sharing plans, and its family plans require each person to manage a separate allotment of data.

The Bellevue, Washington-based carrier has added 3.6 million more monthly subscribers than it has lost so far this year. The company raised its forecast to as many as 4.7 million new postpaid customers for 2014.