NEW YORK – AT&T, the biggest U.S. mobile service, said Tuesday it would offer unlimited mobile phone calls for a flat rate of $99.99 a month, hours after Verizon Wireless announced a similar plan.

AT&T said the new fee option would be available on February 22 and that existing customers could sign up without having to extend their service contract. Verizon Wireless, the No. 2 U.S. mobile service, is a venture of Verizon Communications and Vodafone Group, CNET News.Com reported.

Under Verizon Wireless’s plan, for $99 a month subscribers can use their cellphones to call anyone in the U.S. at any time. AT&T will charge $99.99, with the plan available on all devices and roaming charges being eliminated.

The plans are being hailed as an illustration of how wireless voice service is turning into a staple service, like its old-fashioned tethered long-distance and local voice counterparts. In moving toward its plan, Verizon Wireless — which is jointly owned by Verizon Communications Inc. and Vodafone Group PLC — hopes to win over so-called power users from its rivals.

Verizon Wireless’s move, which it called a “game changer,” may set off cheaper unlimited wireless plans and accelerate the commoditization of wireless voice service down the line.

But wireless carriers have seen their voice revenue growth stagnate amid stiff competition. Instead, players such as Verizon Wireless look to data services such as text messaging, photo transfers and music downloads for growth.

“Voice is the foundation of our business, and will be an integral part of our offer,” said Mike Lanman, chief marketing officer for Verizon Wireless. “Data is clearly emerging as an increasing contributor…to our revenue stream.”

The carrier hopes the plan will attract high-end customers looking for a consistent cellphone bill. Mr. Lanman acknowledged the $99 plan isn’t cheap, and expects “power users” to sign up for the service. Both consumers and business customers will be able to sign up for the plan.

“The market [for high-end users] is getting bigger as people rely more and more on wireless,” he said.

The move breeds customer loyalty and locks them down for longer contracts, said William Ho, an analyst at research firm Current Analysis. Volume subscribers already with Verizon Wireless will have more of a reason to stay, and heavy users on other services may see the unlimited plan as an attractive alternative.

In launching this plan nationwide, Verizon Wireless beat rival Sprint Nextel Corp. to the punch. There have been rumblings that Sprint was considering a similar move to differentiate itself.

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