Fairfax County supervisors threw out a proposal yesterday to charge county residents $200 for ambulance trips starting this year, saying that the elderly and uninsured could be afraid to call for help in an emergency.

But the board, facing homeowners angered at higher taxes, gave itself another possible revenue source to tap should it decide to trim more than the planned 2 cents from the real estate tax rate. The fee that appears on phone bills to defray emergency dispatch costs -- called the E-911 tax -- could rise as high as $3 a month from the current $1.75 if supervisors approve the increase after a public hearing next month.

Also yesterday, advocates for the homeless warned the board to brace for a possible upswing in homelessness as the federal government makes cuts to a program that provides rental assistance to thousands of Fairfax residents.

A reduction in so-called Section 8 vouchers probably would increase the number of homeless families in Fairfax and strand hundreds more on the streets or in shelters unless the county, "through any means at our disposal," creates more affordable housing, said Sharon Kelso, co-chairman of the county's Homeless Oversight Committee.

"The prospect of future federal funding cuts to the county's inventory of . . . housing beds for homeless individuals and families is of grave concern," she said.

The U.S. Department of Housing and Urban Development has proposed reductions of as much as 7 percent in its largest housing assistance program, county officials said.

The cuts would last through 2004. More than 3,140 families in Fairfax rely on the vouchers to make their rent, while 6,532 more are on a waiting list for them.

Hundreds of those families end up in shelters, in transitional housing or on the streets. This year, the committee's annual count found 1,944 homeless people, a 17 percent increase from 1998. More than 60 percent of them were members of families; 758 were children.

Kelso said that the count was taken over a few days in January and that the total number could fluctuate from month to month.

At the crux of the county's homeless problem is the lack of affordable housing, Kelso said. And at the committee's suggestion, Supervisor Catherine M. Hudgins (D-Hunter Mill) challenged members of the Board of Supervisors to create or preserve 10 affordable homes or apartments a year in their districts. The proposal passed unanimously.

The 7 to 3 vote to abandon ambulance charges came as supervisors scheduled public hearings on a series of new or increased fees County Executive Anthony H. Griffin has proposed to raise revenue in a tight budget year. Permits for new wells and sewer hookups, parking violations and boarding at animal shelters would cost more, and cell phone users could be charged a $3 monthly tax on their bills.

An ambulance charge would have generated about $2.5 million a year, and with a cell phone tax would have allowed supervisors to cut a third cent from the current real estate tax rate of $1.21 per $100 of assessed value. But some board members said the extra money would have put their constituents at risk.

"I've had heartburn with this," said Supervisor Penelope A. Gross (D-Mason), whose district includes thousands of immigrants without health insurance. "Do we put them at risk?"

Supervisors voted to convene a task force to look into a fee-for-service ambulance system and reconsider it next year. The county's volunteer firefighters also had expressed fears that a fee would discourage donations to them.

Arlington, Alexandria and the District charge for ambulance trips. In Fairfax, the service would have been provided regardless of ability to pay. The cost of transport would have shifted from the county to health insurance companies and Medicaid and Medicare.

Griffin, in his spending plan for the next fiscal year, proposed increasing the E-911 tax to $2 per phone line. But board members said that by shifting much more of the cost of emergency dispatching to residents through fees, the county could save about $7 million for other uses including tax relief.

The plan to advertise a higher E-911 tax rankled the board's three Republicans and Chairman Katherine K. Hanley (D-At Large), who voted against it.

"That's an incredible amount you're adding," Supervisor Stuart Mendelsohn (R-Dranesville) said. Mendelsohn and Supervisor Michael R. Frey (R-Sully) argued that a tax of up to $3 would unfairly burden businesses, because they use more phone lines than residential customers and the tax applies to each phone line. Businesses also make fewer 911 calls, they said.

Catherine M. Hudgins challenged colleagues on the Board of Supervisors to create or preserve 10 affordable homes a year in their districts.Supervisors voted against County Executive Anthony H. Griffin's proposal to charge county residents $200 for ambulance trips.