AT&T Inc., the second-largest U.S. wireless carrier is introducing service plans aimed at families that use lots of data, cutting about $40 a month from premium users' bills.

Under the new offer, families with four smartphones can pay $160 a month for 10-gigabytes of shared data, unlimited calling and text messaging, the company said in a statement this past weekend. A similar package under AT&T's previous plan would cost $200, and Verizon Wireless charges $260 for the same plan. The offer requires no contract, and phones must be purchased at full price either up front or in installments.

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The move is an escalation of competition in the mobile market where AT&T and T-Mobile U.S. Inc. have run back-and-forth attack ads and offered $450 in credit to entice customers to switch service providers.

"This is about being competitive," said AT&T Mobility chief marketing officer David Christopher. "We feel we have the best network and the best value in the marketplace," Christopher said.

T-Mobile, based in Bellevue, Wash., has gained 2.1 million monthly customers in the past three quarters. This is a reversal of its 2012 performance, fueled by a switch to no-contract plans and quicker phone upgrades and payment financing.

The gains at T-Mobile have helped slow AT&T's user growth. Earlier last week, the Dallas-based carrier said it signed up 566,000 contract wireless customers in the fourth quarter, compared with 780,000 a year ago.

All existing AT&T customers sharing 10-gigabytes or higher will automatically be placed on the new plan, said Fletcher Cook, an AT&T spokesman.