Feb. 19 (Bloomberg) -- AT&T Inc., Verizon Wireless and T- Mobile USA Inc., three of the top four U.S. mobile-phone carriers, announced flat-rate subscriptions for unlimited wireless calls, intensifying pressure on Sprint Nextel Corp.

Verizon Wireless parent Verizon Communications Inc. fell the most in almost six years in New York trading after the mobile carrier announced a plan to give users unlimited calling time for $99.99 a month. AT&T announced an identical service, and T-Mobile offered unlimited text messages and calls for the same price.

The flat-rate plans may force Sprint, which lost 1.2 million customers with wireless contracts in 2007, to offer a similar subscription at a lower price, perhaps $70 to $80 a month, said analyst Christopher King of Stifel Nicolaus & Co. in Baltimore. Verizon, which trails AT&T in total wireless customers, added 1.6 million contract subscribers last quarter.

``This very well could be the beginning of a significant price war,'' said King, who advises holding onto shares of Sprint, AT&T and Verizon Communications. ``Once you have an all- you-can-eat plan out there, voice minutes become a commodity.''

Verizon Communications, which co-owns Basking Ridge, New Jersey-based Verizon Wireless with Vodafone Group Plc, plunged $2.49, or 6.6 percent, to $35.34 at 4 p.m. in New York Stock Exchange composite trading, the biggest drop since July 2002.

AT&T fell $1.99, or 5.3 percent, to $35.89 after announcing its plan would begin Feb. 22. Sprint, the third-biggest U.S. wireless carrier, lost 34 cents to $9.23. T-Mobile parent Deutsche Telekom AG fell 11 cents to 13.13 euros in Frankfurt.

`No Choice'

``Sprint has no choice'' but to offer a cheaper flat-rate plan, said Credit Suisse analyst Christopher Larsen. ``You can't come out and offer the same plan as Verizon. What's disruptive about that?'' The New York-based analyst expects shares of all three U.S.-based carriers to outperform the broader market.

AT&T, which added 1.2 million contract customers last quarter, announced its flat-rate subscription in a statement about five hours after Verizon officially began its plan. The $99.99-a-month rate applies to all handsets AT&T supports, including Apple Inc.'s iPhone.

``We are prepared to respond very quickly to meet customer needs and changes in the competitive environment,'' AT&T spokesman Mark Siegel said.

Customers of San Antonio-based AT&T spent an average of $50.28 a month on wireless service last quarter, while Verizon's spent $51. Sprint plans to announce full results for last quarter on Feb. 28.

T-Mobile Plan

T-Mobile's $99.99-a-month unlimited plan, which begins Feb. 21, will cover all types of mobile messages, including pictures, in addition to voice calls, company spokesman Peter Dobrow said today. Equivalent plans from AT&T and Verizon Wireless would cost about $120 a month.

Sprint, based in Overland Park, Kansas, has tested a flat- rate plan in four U.S. cities, selling calls, text-messaging and Internet access for $119.99 a month. Verizon matched that price today for the same services and began selling a plan that includes mobile video and driving directions for $139.99.

Sprint is still evaluating the results of the trial, spokeswoman Emmy Anderson said.

The flat-rate plan is aimed at Verizon's most frequent mobile-phone users, about 15 percent of its subscriber base, Jennifer Fritzsche of Wachovia Capital Markets said in a research note today.

The price should help retain more subscribers, said the Chicago-based analyst, who expects Verizon Communications shares to perform in line with the rest of the market. Verizon Wireless had the industry's lowest customer turnover rate last quarter at 1.2 percent, up from 1.14 percent the year before.

13 Percent

Verizon's flat rate may appeal to less than 13 percent of subscribers, Stifel's King estimated. While the plan probably won't hurt profits at current prices, investors don't know how the shift will affect the industry, he said.

``The industry's future is more clouded, at least from a wireless perspective, than it was,'' King said.

Verizon may give up $93 million a month in sales as subscribers who spend more than $100 a month for voice calls switch to the new plan, said Banc of America Securities analyst David Barden in a research note today. The wireless company's sales rose 15 percent to $43.9 billion last year.

The company could make up some of the lost sales by persuading customers on an $80-a-month plan to upgrade, said New York-based Barden, who recommends buying Verizon Communications shares.

Data Services

Wireless carriers have become more dependent on data services such as text messaging and ring tones for revenue growth as prices for voice minutes have fallen. The industry's average revenue per minute of voice use fell 5 percent in 2006 to 6 cents, according to a U.S. government report released this month.

More than four-fifths of people in the U.S. already have a mobile phone, according to the Washington-based industry group CTIA-The Wireless Association. With fewer new users to attract, providers may turn to flat-rate pricing as competition intensifies, Wachovia's Fritzsche said.

To contact the reporter on this story: Crayton Harrison in Dallas at tharrison5@bloomberg.net.

Last Updated: February 19, 2008 16:42 EST