AT&T Inc. is cutting mobile-phone subscription charges for the second time in two months, seeking to hold on to smartphone customers amid a price war with T- Mobile US Inc.

Starting Sunday, AT&T customers can sign up for a Shared Value plan with 2 gigabytes of data for $65 a month, down from $80. Adding a second phone to the plan brings the price up to $90, down from $105. The offer applies to both new and current customers, though existing subscribers will have to sign up again to qualify, said Mark Siegel, an AT&T spokesman.

The price reduction came a day after T-Mobile said it will double to 1 gigabyte the amount of data alloted to its $50-a-month Simple Choice package. AT&T cut its shared data plan for families by $40 last month as it seeks to keep pace with T- Mobile’s aggressive pricing strategy. T-Mobile gained 2.1 million customers in the past three quarters, reversing its performance in 2012. The company offers a plan with 2.5 gigabytes of data for $60 a month for one phone or $90 a month for two phones.

Over the past year, Bellevue, Washington-based T-Mobile introduced a series of pricing changes to shake up the industry, becoming the first carrier to offer customers quicker phone upgrades, payment financing, international data and text- messaging at no extra charge. It also started a $450 credit program for users who switch from other providers.

AT&T, based in Dallas, is the second-largest U.S. wireless carrier, trailing Verizon Communications Inc. T-Mobile is fourth, behind Sprint Corp. Competition between AT&T and T- Mobile, which abandoned a merger attempt in 2011, has intensified in part because they use similar technology, making it easier for customers to switch devices between their networks.