AT&T announced today a new plan feature for its mobile share value customers: shareable rollover data. The carrier says more than 50 million subscribers will automatically gain the benefit when it kicks off at the end of the month on January 25th. If this news sounds familiar, that’s likely because competing wireless provider T-Mobile became the first of the big four national carriers to offer rollover data with its “data stash” feature announced last month alongside a free 10GB data promo. Here’s how AT&T says its version of rollover data works:

All levels of AT&T Mobile Share Value plan customers automatically receive the Rollover Data feature – no signing up, no additional cost.

Rollover Data can be shared with everyone on the same Mobile Share Value plan.

If you do not use your full allotment of plan data in a given month, the extra amount automatically rolls over and is available to be used within the next month.

Within a given month, you will use your plan allotment first, before you begin using your Rollover Data. Unused Rollover Data does not carry over.

That means AT&T’s rollover data feature is a bit tighter than T-Mobile’s data stash feature; AT&T is limiting rollover data to the following month only—presumably to avoid subscribers accruing large data pools—while data stashed with T-Mobile can’t be stored for longer than 12 months.

The rollover data feature should help some customers avoid overages or have more data to spare on any given mobile share plan, though.

On a 10GB mobile share plan with 8GB used in a service period, for example, the mobile share plan would then have access to a total of 12GB the following month. The limitations come into play quick, though, as the first 10GB would be used first the following month with the extra 2GB expiring the next month. If only 7GB of data of data were used the next month, though, then 13GB would be available the following month.

While the rollover data feature requires a mobile share plan, the rollover data applies to each device including smartphones and tablets including the iPhone and iPad. AT&T says both family and business mobile share plans will have access to the feature automatically starting on January 25th with the extra data being trackable through the carrier’s site and mobile app.

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