Kathleen Leroy has 10,000 phone numbers in her files at All Saints' All Day child-care center in Bethesda. On Oct. 1, when the new telephone dialing system goes into effect for the Washington metropolitan area, many of them will no longer work.

As of Oct. 1, for the first time calls between the District, Maryland and Northern Virginia must include the area code. Millions of local calls that formerly required only seven digits will now require 10.

"We have a lot of parents who live in Maryland but work in D.C.," said Leroy, who is reviewing all of the center's emergency numbers. "It's a big headache, a real nuisance."

For the 1.9 million Chesapeake & Potomac Telephone Co.'s customers in the Washington area, the cost of the new dialing system ranges from added aggravation to expensive reprogramming of burglar alarms to worry that interstate fire and rescue operations may be slowed.

The phone company -- which spent weeks reprogramming computer software for the Pentagon, CIA and other government agencies -- said the change was necessary because it ran out of telephone numbers.

The shortage of unassigned numbers is so acute in the District that not a single number would be left by "this October or November" without the new system, according to C&P spokesman Michel Daley.

Facsimile machines, computers, pagers, and car and home phones have resulted in soaring requests for new telephone numbers -- 90,000 in the Washington region last year alone.

The new 10-digit dialing system is one of the biggest changes for the region's telephone users since 1959. That year for the first time, Washington area residents were able to dial long-distance numbers without an operator.

The new system works this way:Local calls between jurisdictions will not be connected without the proper area code: 202 for the District, 301 for Maryland and 703 for Northern Virginia. For example, a caller dialing Prince George's County from the District must now include Maryland's 301 area code before the seven-digit number. Likewise, a call from Arlington to the District requires the city's 202 area code. If a caller does not dial the area code, a recorded message is supposed to suggest hanging up and trying again with the additional three digits. Dialing the number 1 before the area code is only necessary if the call is long distance. If the number 1 is dialed, though, the call will go through and not be billed as long-distance. All calls that are now considered local will remain local as far as the phone bill is concerned. In making local calls within the District, within Maryland and within Virginia, the area code will not be needed.

The expansion to 10-digit dialing is expected to create 8 million new numbers in the region. However, because the vast majority of them will be in the District and Virginia, Maryland is expected to run out of numbers again in two years.

All of Maryland currently falls under the 301 area code, and C&P said it will split the state into two area codes in 1992. C&P spokesman Richard Ellis said Maryland's area code changes are not being made now because "we don't want to confuse people more than we have to."

The change that is happening Oct. 1 has big and small implications for almost everyone who uses a phone.

Speed-dialing phones must be reprogrammed, billboards repainted, business cards reordered. Some companies, worried that the change will hurt business, are ordering phone lines in each jurisdiction so that the different area code won't scare off potential business.

"You don't need a space probe to get here, but some people in Maryland think Alexandria is as far away as the moon," said Barry Mudd, an owner of the Capital Entertainment booking agency in Alexandria.

New phone books will list a Bethesda number for Mudd's firm even though he doesn't have a storefront there (the call will roll over to Alexandria). He explained: "It's a defensive measure. I don't want the different area codes to add to the perceived distance. I don't want this to hurt sales."

Real estate agents, whose phones are ringing less these days because of a softening housing market, are scrambling too, redoing newspaper advertisements and "For Sale" yard signs to make sure telephone technicalities don't hurt sales further.

Long & Foster Realtors is immediately altering 3,000 "For Sale" signs in yards near jurisdictional boundaries, such as houses on Western Avenue. The north side of the street is in Maryland with a 301 area code; homes on the south side use the District's 202 area code.

Timothy Healy, a Long & Foster agent, said it's been calculated that a real estate agency pays about $100 for each incoming call -- figuring the costs of advertising, office space and licensing.

"Obviously," he said, "calls are of major importance to us and we don't want people unable to get through to us because of an area code mix-up."

Edward Sherburne, assistant chief of the Bethesda-Chevy Chase Rescue Squad, doesn't like to think that some people might soon get a recording when they're trying to reach one of his ambulances.

Thousands of Northwest Washington residents rely on the Bethesda-Chevy Chase volunteer rescue squad for ambulance service and have long kept its 652-1000 number handy. After Oct. 1, Maryland's 301 area code must be dialed before that number. Otherwise, the call won't go through.

"I can't believe that for fire and rescue as well as money -- we offered to pay -- they {C&P} wouldn't let us keep our number," Sherburne said. "It's incredible. In our business, delay and confusion can cause major problems."

C&P said exceptions could not be made under the new system, not even for the Defense Department.

Although the Pentagon is on the Virginia side of the Potomac River, long ago its telephone exchanges were assigned the District's 202 area code. After Oct. 1, calls to the Pentagon must include Virginia's 703 area code.

"I imagine there will be some initial confusion" at the office complex wired with more than 100,000 miles of phone cable, said Pentagon spokeswoman Susan Hansen.

The CIA in Langley has also been severed from the 202 area code, but spokesman Mark Mansfield said details about the intelligence agency's phone system are secret. Rest assured, though. He said the "software changes will be activated the night before" Oct. 1.

Metropolitan regions around the country -- including Los Angeles, Boston and Chicago -- also have recently been divided into different area codes, with a chorus of complaints from businesses. Currently, officials in Beverly Hills, Calif., are feuding with Pacific Bell over its plans to add a third Los Angeles area code whose dividing line would run through their wealthy neighborhood.

By mandating area codes in the Washington area, the phone company triples the available numbers, since identical telephone numbers can then be used in Maryland, Virginia and the District.

For Honeywell Protection Services, the new phone system forced the company to reprogram burglar alarms in 1,000 houses throughout the region.

"It's creating a lot of work for us," said Honeywell operations manager Richard Ptomby. Customers weren't thrilled either; they each had to pay $53 to adjust their home security systems, which are essentially automatic-dial telephones that alert a central office of a possible break-in.

The new phone system also will affect people who work in one jurisdiction and live in another. The phone company is reminding parents to tell their children about their phone number changes. The children may also have to remind their parents.

As of Oct. 1, it will take Ed Barba less time to travel to the District than to call there.

"We pride ourselves on being able to deliver a television to the District in two seconds -- the District line is right outside our door," said Barba, manager of Royce's TV Shop on Eastern Avenue in Silver Spring.

"Now it takes longer to call our D.C. customers than to get to D.C. This is yet another hassle to put up with."