Some calls to 911 in Fairfax County have taken longer than usual to answer in recent weeks because of problems in converting the dispatch system at the new communications center from a manual to a computer-assisted operation.

A Herndon resident said that she dialed the emergency telephone number last week after she began having severe chest pains but that "no one answered the phone." Another county resident said that he tried to report a stolen car and that the line went dead after ringing several times.

County officials acknowledge that there have been delays but said that they have been minimal and that improvement has been made as the operators become more familiar with the complex system. The officials emphasize that, to their knowledge, the delays have caused no death or injury.

Richard A. King, deputy county executive for public safety, said the computer-aided dispatch system has been "shadowing" the manual system and is not scheduled to become the primary system until tomorrow.

Nevertheless, sources familiar with the new system are concerned, saying that some people have experienced exceptionally long delays or have not been able to get through on the emergency number.

For example, Mary Johnson, a 28-year-old Herndon resident, reported that the phone rang "five to 10" times when she dialed 911 because of chest pains. She said she hung up and redialed, letting the phone ring about the same number of times. Still getting no answer, Johnson said, she panicked and dialed the operator.

"I was afraid -- I was at home by myself with my two kids," said Johnson. "The pain was very intense. I thought maybe I was going to pass out."

Johnson said that after she called the operator, help came quickly, and she recovered. Johnson said rescue workers told her that about 10 other people had reported problems with 911.

Another Fairfax resident, Paul Rosa, said he tried to report what appeared to be a stolen car on June 29, the day after the new system began phasing in. Before calling the emergency communications center, Rosa said, he tried the McLean district police station and was given a phone number that he was told would connect him with a dispatcher. When he dialed the number, he got a recording. Then he tried 911.

"I called 911, and it rang, oh, I'd say six times, and the phone simply went dead. Then I called 911 again, and the same thing happened."

That same evening, nursing home officials tried without success to reach the emergency center on a nonemergency line. The phone rang "for an extensive period of time," a nursing home official said. They later got through on another line.

In another incident, a car fire was reported by calling the fire station directly after the caller could get no answer on 911, according to a source.

King said that for a county that handles about 1 million emergency and nonemergency calls a year, the problems in recent weeks have been "minimal to none." He said it is too soon to judge the highly sophisticated system.

Lt. Col. Michael W. Young of the county Police Department, which has operational control of the new public safety communications center, said that staff adjustments have been made and that the time it takes to answer calls has been slashed "drastically" since the system's debut. During peak hours, he said, calls are being answered on average in 5.1 seconds.

Also, sources say that in the past few weeks there have been troubling delays in dispatching calls once they are received. A fire official said that it has taken as long as six minutes and that "that's absolutely intolerable."

Young disputed that figure but agreed that dispatching has taken longer. He said, however, that the new computer-aided system is supplying a raft of information not previously available.

Fire Chief Warren Isman said he will become concerned if delays in dispatching continue after the new system is completely phased in. "We have taken a major step in a technological age . . . . As a result, there is a learning curve which we absolutely need to go through," Isman said. "There has been a little time added to the dispatching but certainly not overwhelming."

If all goes according to plan, officials said, the system will be a technological marvel and will make the county's new emergency center in the police department's Pine Ridge facility even faster and more efficient. The center was formerly known as the emergency operations center and housed in the basement of the Massey Building in Fairfax City.

The system provides call-takers with a visual display of the phone number and the address and automatically verifies information, assigns a priority to the call and relays the information to the appropriate dispatcher.

The computers, which give the center the feel of Mission Control at Johnson Space Center in Houston, will do such things as locating the nearest cross streets to the reported event and displaying them on screens.

During the transition, residents should try to stay on 911 rather than hanging up and calling the operator, said King and Young, who urged residents to report any delays.