AT&T rollover dataAT&T is taking a page from T-Mobile’s Un-Carrier book, announcing a new program today that will enable its customers to save unused data for use later.

But unlike T-Mobile’s plan, or even AT&T’s current rollover plan for voice minutes, this one comes with a catch: Data has to be used within the next month, and won’t pile up month after month like you may expect.

AT&T said that beginning Jan. 25 all customers on a Mobile Share Value plan — or more than 50 million customers — will be able to save any unused data for the following month only.

This is how it works: If you have a 15GB and only use 10GB in a given month, you’ll roll over 5GB and have a total of 20GB available to use within the next month. If you were to only use 10GB in the second month, you’ll again roll over 5GB and have a total of 20GB available the next month.

In other words, you would not have 30 GB to use in the third month, despite having 10GB of unused data from the two prior months.

The announcement is no doubt in reaction to T-Mobile’s Un-carrier 8.0 announcement made in December. The new program, called “Data Stash,” lets its customers roll over their unused data each month into an allotment that they can use for up to a year. T-Mobile kicked off the program by adding 10GB to everyone’s stash.

T-Mobile’s CEO John Legere was quick to point out the differences between the two plans on Twitter.

The wireless industry has always copied each other, quickly responding to the competitor’s new offers and rate plans. Customers typically benefit, especially if they switch carriers. AT&T’s move today may attract new customers, but it will also help with retention.

That’s especially important since T-Mobile seems to be benefiting by being first to roll out some of these new offers. Today, T-Mobile announced preliminary results for the fourth quarter, disclosing that it added 2.1 million customers during the period, capping its biggest year ever.

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