Shares of Washington's newest telephone company, TeleCorp PCS Inc. of Arlington, are up more than 70 percent in their first two days of trading, boosted by the company's role as a key supporting player in the aggressive move into wireless communications by AT&T Corp.

TeleCorp raised $207 million in its initial public offering, which hit the market Tuesday just as details of AT&T's new wireless strategy leaked out.

The nation's largest long-distance company plans not only to expand its mobile phone service but also to use wireless technology to provide phone and Internet service to homes and businesses, bypassing the wires of local phone companies.

TeleCorp founder and chief executive Gerald T. Vento said his company is one of three regional partners that will help AT&T expand its mobile phone service into smaller cities.

AT&T is also the largest outside stockholder in TeleCorp, which has licenses to provide mobile phone service in 21 markets, including New Orleans; Memphis; Little Rock; Cape Cod, Martha's Vineyard and Nantucket, Mass.; Puerto Rico; and the U.S. Virgin Islands.

The firm's IPO was greeted on Wall Street with the kind of welcome once reserved for Internet stocks. The shares jumped from their $20 offering price to $33 on their first day of trading Tuesday and gained an additional $2.50 yesterday to close at $35.50.

Salomon Smith Barney Inc., the lead manager of the IPO, had estimated that TeleCorp's stock would bring $16 to $18 a share but priced the stock at $20 because of heavy demand from investors.

TeleCorp sold only 10 percent of its shares in the IPO, which means the entire company is worth $3.3 billion at yesterday's market price.

With the mobile phone market growing 40 percent this year, the burgeoning companies in the business have become some of Wall Street's favorites.

The stock of Omnipoint Communications Corp. of Bethesda cracked the $90 barrier Tuesday and closed yesterday at $90.43 3/4, more than 10 times its price a year ago. Shares of Reston-based Nextel Communications Inc. are going for almost six times their price of last December, closing yesterday at a record $116.06 1/4. The stock gained more than $17 yesterday after Nextel won a court case that could allow it to be additional frequencies.

TeleCorp is the largest of the three regional AT&T partners. Pennsylvania-based Triton PCS Inc., which has licenses in several states in the Southwest, went public Oct. 28, and its shares have nearly doubled since then. The stock, first offered at $18 a share, closed yesterday at $36.12 1/2. Tritel Inc., based in Mississippi, filed a $172.5 million IPO proposal last Friday to raise funds for mobile phone systems in the Southeast.

With the market for mobile phone stocks so hot, TeleCorp's underwriters completed the IPO just five weeks after the first draft of the offering plan was filed with the Securities and Exchange Commission. Most IPOs take 2 1/2 to three months to bring to market and longer timetables are not unusual for start-up companies.

Like the other two AT&T partners, TeleCorp operates under the SunCom brand and uses the AT&T globe logo. Under the partnership agreement, SunCom customers will connect to the AT&T PCS network when they roam outside their hometowns.

AT&T was an early investor in the company and purchased an additional 2.2 million shares this week, keeping a 17.8 percent stake.

Vento, 52, a former vice chairman of Sprint Spectrum/American PCS, co-founded TeleCorp in 1996 with executive vice president and chief financial officer Thomas H. Sullivan, 37, a communications lawyer who served as counsel to several mobile phone services.

Vento holds 5.1 million shares of stock and options, worth $181 million at yesterday's closing price. Sullivan has 3.3 million shares and options, worth $117 million. But the co-founders hold 52 percent of the voting power in the company through agreements with some of their big investors.

Vento said the company bought the rights to offer mobile phone service in some of its markets at Federal Communications Commission auctions but became a much more significant player when it hooked up with AT&T last year and acquired licenses for some additional markets.

Wall Street analysts said not even AT&T is big or rich enough to keep up with the exploding demand for mobile phone business. Industry sources speculate that AT&T may buy out its junior partners at some point. That potential is another reason the firms' stocks are so popular, said one analyst, who asked not to be named.

Before this weeks IPO, TeleCorp had already raised $1.4 billion in capital, via junk bonds, loans and financing from equipment suppliers.

The company put its first phone into use in New Orleans in February and is already on the air in all of its other markets. So far the network is capable of reaching about three-quarters of the 16.5 million people who live in its service area.

Since February, TeleCorp has generated $48 million in revenue, about $19 million of it from roaming fees paid by AT&T customers using their phones in TeleCorp's service area. The company posted a loss of $144 million for the first nine months of the year.

CAPTION: Gerald T. Vento is the founder and chief executive of TeleCorp.