By Thomas Gryta 

Taking a page from AT&T's history book, T-Mobile will let subscribers roll over their unused wireless data each month, its latest move in the industry's escalating price war.

The move takes aim at efforts by market leaders AT&T and Verizon to charge their subscribers more as their data use goes up. Both carriers have moved their customers away from unlimited data plans to ones that get more costly as data allotments increase, a key to their plans for squeezing more growth out of a market where most people already have smartphones.

The move harks back to an earlier era when cellphone users had to carefully keep track of their voice minutes. At the time, one of AT&T's predecessor companies, Cingular Wireless, capitalized by letting subscribers roll over their voice minutes.

T-Mobile's announcement Tuesday comes a week after AT&T and Verizon warned that the cost of keeping up with rivals' promotions is hitting their bottom lines. Competition from T-Mobile and more recently Sprint is making them work harder to keep their wireless subscriber counts growing.

In October, T-Mobile raised its projection for postpaid net additions for 2014 to a range of 4.3 million to 4.7 million, up from a previous estimate of 3 million to 3.5 million. The carrier has added more than 5.6 million customers since the beginning of 2013, in a saturated industry with little real subscriber growth.

The carrier's latest move, called Data Stash, lets people roll over their unused data for up to a year beginning next month. The plan applies to postpaid customers on the company's Simple Choice plans who have a smartphone plan with 3 gigabytes or more of high-speed data, or a tablet plan with 1 gigabyte or more.

T-Mobile is starting people with 10 gigabytes per line in their reserve, and data begins rolling over when that initial allotment is used.

AT&T, Verizon and Sprint offered subscribers extra data on many plans earlier this year as a way to make them more attractive for subscribers without cutting prices further. In many cases, the plans offered far more data than typical subscribers use.

Write to Thomas Gryta at thomas.gryta@wsj.com

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