On the heels of a similar announcement from T-Mobile, AT&T today announced that it too will offer rollover data.

Starting Jan. 25, new and existing customers on an AT&T Mobile Share Value plan will see their unused data automatically roll over to next month.

"Rollover Data is an added benefit of being an AT&T Mobile Share Value customer and it's just another way that we're saying thanks to our more than 50 million plus Mobile Share Value subscribers," Glenn Lurie, President and CEO, AT&T Mobility, said in a statement. "We're providing even more value and flexibility, and the best part is it's simple, shareable and easy to track for our customers."

Everyone on a Mobile Share plan, regardless of which level, will get the rollover data perk, and this excess data can be used by anyone on the plan.

You can't just bank all your unused data for a rainy day, though. If you go into a new month with unused data, AT&T will first tap into your monthly allotment before draining your unused data. If you don't use your rollover data the next month, it goes away.

"As an example: If you have four lines and have a 15GB AT&T Mobile Share Value Plan 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," AT&T said. "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."

Keep track of Rollover Data balances on the myAT&T app or online at att.com.

If you need a new device to access that data, meanwhile, AT&T said today that it will be the exclusive provider of a newly designed BlackBerry Passport with rounded edges. The BlackBerry Passport has a large, 4.5-inch square touch screen and a touch-enabled QWERTY keyboard, and will be $199.99 with a two-year agreement from AT&T.

In mid-December, T-Mobile announced that going forward, any high-speed data its Simple Choice customers don't use in a given month will automatically roll into a personal "Data Stash," which is accessible for up to a year. Every Data Stash comes pre-loaded with 10GB of 4G LTE data for free; data does not start rolling over until that 10GB has been used up.

Not one to mince words, T-Mobile CEO John Legere was quick to criticize AT&T's rollover plan as a "bad imitation."

.@WaltBTIG it's not anything close .It's like a short extension and then the same carrier take back...as predicted, bad imitation + trickery

— John Legere (@JohnLegere) January 7, 2015