AT&T Inc., the second-largest U.S. wireless carrier, will offer smartphone and tablet users shared-data plans starting at $85 a month beginning in late August, following a similar move by Verizon Wireless last month.

The new AT&T Mobile Share plans include unlimited calling and messaging and let customers combine as many as 10 devices under one monthly data allotment.

Verizon started a similar plan starting at $90 last month.

AT&T and Verizon Wireless are pushing a shared-plan approach to encourage customers to use more devices and ultimately buy bigger packages of data. By bundling services onto one bill carriers are trying to stop erosion of calling and messaging revenue as consumers who call less and use free messaging services migrate to cheaper packages.

“These aren’t entry-level price plans,” said Roger Entner with Recon Analytics. “These are forward-looking plans for people with multiple devices – the heavy data users of today, who will be more common down the road.”

The plan starts with $40 for 1 gigabyte of data and $45 per smartphone, AT&T said in a statement. The next tier is $70 for 4 gigabytes of data and $40 per smartphone, 6 gigabytes costs $90 and each smartphone is $35, 10 gigabytes is $120 with $30 per smartphone, 15 gigabytes is $160 with a $30 smartphone charge and the maximum is a 20 gigabytes for $200.

Users can add phones that only make calls and texts for $30 a month, laptops for $20 a month and tablets for $10 a month.

More Choices

The idea is to offer consumers more choice, said David Christopher, chief marketing officer for AT&T Mobility. “The future is all about connecting mobile devices to the Internet and we’re making it easier to do that,” Christopher said in a telephone interview.

AT&T says isn’t discontinuing its existing individual and family plans.

The shared-data offering also includes mobile hotspot and tethering features that allow devices to share their data connections.

Verizon introduced its “Share Everything” plans on June 28, replacing nearly all of its traditional phone plans. AT&T says its “Mobile Share” plans will debut in late August, but it is keeping its current individual and family plans.

AT&T is the country’s second-largest cellphone company, behind Verizon. AT&T, which is based in Dallas, has 76.8 million direct wireless customers, and millions more through wholesale arrangements.

AT&T’s and Verizon’s shared-data plans are intended to stimulate the adoption of non-phone devices such as tablet computers and USB modems for laptops by making monthly service cheaper. Analysts believe Verizon’s plan will reduce its service revenue in the short term but pay off in the long term as families add more devices.

SIM Cards

AT&T faces a problem in that subscribers can move SIM cards, or subscriber identity modules, between devices. The SIM card identifies the devices to the network, and in the case of phones, contains the phone number. Subscribers will be tempted to move a SIM card from a basic phone, for which they pay $30 per month, to a smartphone, which can cost as much as $45 per month.

AT&T’s Christopher said the company can tell which type of device a card resides in, but didn’t specify how the company would handle wandering SIM cards.

“If a customer moves SIMs, we’ll have to look at those situations and act appropriately if that happens,” Christopher said.

SIM cards are rare in Verizon devices.

(Bloomberg and The AP contributed to this report.)