Bill Collins swiveled in his leather chair to answer the ringing phone on his cluttered desk at the real estate service firm Cassidy & Pinkard. "Yes, yes. You're the next offer in the queue," he told the caller. "Hold on."

His secretary buzzed him to let him know that one of the biggest developers in town -- Raymond A. Ritchey, executive vice president of Boston Properties Inc. -- was on the other line. "Can you put him on hold?" he asked her, as he spun in his seat to answer his cell phone while balancing the black receiver on his shoulder.

Just then his brother, Paul, whose office is next to his, tapped on the door.

"Bill, I'm heading out to the burbs," he said. He too had just juggled a flurry of phone calls. "I'm doing one out there today," he said, meaning he was closing on the sale of an office building.

It was barely 10 a.m. and the two brothers already were immersed in the day's wheeling and dealing to sell office buildings. The pair has ranked among the top 10 sales brokers in the D.C. region for at least the last four years, according to the Greater Washington Commercial Association of Realtors, an industry trade group.

Last year, the brothers sold about $1.4 billion of the $6 billion worth of the commercial real estate sold in the Washington area. That put them in first place among about 100 brokers, the trade group said. This year, the Collins brothers said they expect to top that amount because they are selling some of the biggest and most expensive buildings in the region, including 1900 K Street and 1001 Pennsylvania Ave.

Real estate brokers are divided into three categories: those representing tenants looking for space, those representing landlords who want to fill their buildings with renters and those representing owners who want to sell. The latter is what Bill and Paul Collins do.

There aren't many siblings who work as a team, industry experts said.

Bill and Paul say they rarely make a deal without consulting each other. "You know each other," Paul said. "You can count on each other."

Bill, 48, has short, spiky hair, a salt-and-pepper goatee, and is built like a football player. Paul, 43, is slightly taller and thinner.

"They have a unique ability to facilitate a sales transaction that results in both the buyer and seller feeling like they got a great deal," said Ritchey, who has done half a billion dollars worth of transactions with the Collins brothers in the past decade. "They've been on our side of the table and across from us in deals, and in every instance I felt good about the outcome."

The Collins brothers compete with a small group of individuals, largely men, who sell buildings in the Washington area worth $50 million or more. "Some years we're on top of them; and some years they're on top of us," said John E. Duffy, a senior managing director at Holliday Fenoglio Fowler LP , one of the largest commercial real estate companies in the country. Duffy and his partner, Stephen C. Conley, also rank consistently in the top 10 sales brokers in the Washington region.

"We're basically friends," Duffy said of the Collins brothers. "I don't think New York is like this. Our kids play on the soccer teams together. We all live within five miles of each other" in Bethesda.

"They're candid; they're honest and they're real," said Nicholas C. Pappas, who is also a top-selling broker in the D.C. region and is a managing director in the Washington office of Eastdil Realty Inc., a major New York real estate firm. "And I think they've got a self deprecating attitude where they don't take themselves too seriously."

In selling buildings, each brother has his own niche.

Bill mainly sells buildings in downtown Washington, while Paul sells buildings in the Maryland and Virginia suburbs. They each carry a legal-size pad that lists on one page the deals they're working on.

The brothers display their success on the windowsills in their offices. The addresses of buildings they've sold are engraved on wood and glass plaques. There is 1275 K Street NW, which went for $77 million this year, Potomac Tower in Arlington, which sold for $106 million; 1615 L Street NW, sold for $124 million; and a portfolio of buildings in suburban Maryland along the Interstate 270 corridor, which sold for $123 million.

They seem to be everywhere: at cocktail parties, dinners and charity golf outings populated by developers and clients. Like other brokers in the region, the Collins brothers have benefited from the Washington area's strong real estate market, which has attracted investors from across the country and overseas.

Several competitors have tried to woo them with expensive dinners, promises of bonuses and higher salaries.

"They have fantastic contacts on Wall Street and with institutional owners." said John D. Lesinski, executive vice president and managing director at Grubb & Ellis Co.'s Washington area office. "From a competitor standpoint, it's frustrating. That's why you find competitors saying, 'I'd rather have them inside my tent then outside.' "

The brothers say they have no interest in leaving Cassidy & Pinkard. "Sure, we get other offers," Bill Collins said. "They're always there, until they see how expensive it would be to [lure] us," he said.

The brothers are fourth-generation Washingtonians. Their great-grandfather, who came from Ireland, settled in a rowhouse near Union Station and worked as a railroad engineer. Bill and Paul Collins, who have three other siblings, grew up in Montgomery County and graduated from Winston Churchill High School. Their father built houses in the Maryland and Virginia suburbs.

The brothers graduated from the University of Maryland at College Park. Bill studied business and finished in 1978; Paul also majored in business and graduated in 1983.

Bill moved to Houston, and traveled to Europe and the Middle East to buy and sell pipes, valves and generators for a Saudi family. He joined Cassidy & Pinkard in 1983 and made a name for himself assembling sites in the East End of the city for major developers to buy.

After college, Paul took a job with real estate service firm Spaulding & Slye, representing tenants looking for space in Northern Virginia. He joined his brother at Cassidy & Pinkard in 1990 and the pair became a sales team, with a staff of 18.

They spend their time putting together deals and wooing new clients and investors. At a recent early-morning briefing, Bill tried to interest German investors.

"They call Northern Virginia the death sciences, for the defense-related contractors out there," he said to the half-dozen men in business suits sitting around a conference table. "Maryland is the life sciences with biotech companies and the National Institutes of Health. And D.C., you can just call that government."

He ran through a whirlwind of Washington history, from the 1968 riots to the recent building boom near MCI Center and East End. He said Maryland and Virginia had been hit hard by the technology downturn but are reviving because of the government contracting money pouring into the region.

He added a note of caution that yes, the buildings he showed in a PowerPoint presentation were fetching high prices. "It's all about supply and demand," Bill told the group. "We've got more money chasing fewer deals."

The group told him they wanted higher returns. "If you want yield, you've got to go outside the Beltway," Bill said. The Germans and their advisers nodded. They shook hands with Bill and left.

Paul later asked him how it went. Bill responded: "We'll see."

Paul Collins, left, and his brother, Bill, accounted for about $1.4 billion of the $6 billion worth of the real estate sold in the region last year.