BOSTON - Harvey Collier, a mortgage broker in Fort Lauderdale, Fla., says he gets as many as 10 calls a month from people planning to default on their loans. The twist: They first want financing to buy another home. Real estate professionals call it 'buy and bail.' This involves acquiring a new house before the buyer's credit rating is ruined by walking away from the old one because it's 'underwater,' or worth less than the mortgage. It's an attempt to escape payments on a home whose value may never recover while securing a new property, often at a lower price with a more affordable loan. The practice, which constitutes fraud if borrowers lie on loan applications, is continuing even after Fannie Mae and Freddie Mac, the biggest U.S. mortgage- finance companies, beefed up standards to prevent it, according to brokers such as Collier and Meg Burns, a staffer at the Federal Housing Finance Agency. Whether driven by greed or desperation, the persistency of buy and bail underscores the lingering impact of the worst housing crash since the Great Depression. 'People were holding on, hoping the market would turn around,' said Collier, who won't work with applicants who intend to go into foreclosure. 'But now they're giving up because there's no light at the end of the tunnel in places like Florida.' The value of U.S. homes fell by a third from 2006 to 2009, as tracked by the SP /Case-Shiller index. In some areas, the losses were bigger. Prices declined 56 percent in Las Vegas, 55 percent in Phoenix and 49 percent in Miami. Such declines have left more than a fifth of single-family homeowners with mortgages underwater in the second quarter, according to a report this week by Zillow.com, a Seattle-based data company. About 12 percent of residential-loan defaults in February were strategic, meaning homeowners decided not to make payments even though they could afford to, New York-based Morgan Stanley said in an April report. The rate, which was about 4 percent in mid-2007, probably will increase even if home values start to recover, said Frank Pallotta, managing partner of Loan Value Group, a mortgage- consulting firm in Rumson, N.J. 'After home prices bottom, the borrower in a position of negative equity is able to quantify exactly how long it will take to recoup the loss, and may decide to walk away,' Pallotta said. Most likely to walk away are borrowers with the best credit scores and so-called jumbo loans that exceed the caps set for mortgages bought by Fannie Mae and Freddie Mac, which range from $417,000 in most locations to $729,750 in high- cost areas, according to the Morgan Stanley report. People who choose to default typically have lost $100,000 or more in property value, said Brent White, a law professor at the University of Arizona. No data exist on strategic defaults done in tandem with buy-and-bail purchases. Buy and bail is most often pursued by people with big enough paychecks and low enough debt to qualify for two homes, according to Mark Goldman, a broker at Cobalt Financial Corp. in San Diego. That threshold is easier to meet since home prices retreated and mortgage rates fell to an all-time low, he said. The average U.S. rate for a 30-year fixed home loan dropped to 4.49 percent, the lowest in records dating to 1971, Freddie Mac said this month. 