Fairfax County officials said yesterday that their new and occasionally dysfunctional 911 emergency communications center was operating properly, with calls being answered in less than a second.

Mike Fischel, director of the Public Safety Communications Center, said that American Telephone & Telegraph Co. workers had discovered and repaired a problem in the telephone system that a week earlier caused it to shut down for 30 minutes.

Because of a "bug in the software," Fischel said, employes were able to receive calls at the communications center July 24 but could not not hear the caller. Because of a backup system, however, there were no delays in providing emergency service, he said.

Although there have been "persistent problems" with the telephone system, including telephone sets overheating and some background noise, the computer-aided dispatch system has performed well since it became the primary system July 23, Fischel said.

The new system cost $12 million and is called CAD, for Computer-Aided Dispatch. It is housed in the police department's Pine Ridge facility in central Fairfax and somewhat resembles Mission Control at the Johnson Space Center in Houston. Red numbers flashed on a wall show how long calls go unanswered.

Lt. Col. Michael W. Young of the Fairfax Police Department, which operates the communications center, said most calls were being answered in "less than a second, almost immediately."

The new system displays the phone number and the address, then verifies the information, assigns a priority to the call and relays the information to the appropriate dispatcher -- police, fire or ambulance.

During a month-long transition to CAD, there were complaints of delays both in answering 911 calls and rerouting them to the proper dispatcher. Officials said those problems were because of the complexity of the system and the fact that employes were initially doing double duty by monitoring the old system to check the performance of the new one.

County officials said they had expected some problems with the new system, but did not expect them to last this long. They said, however, that other law enforcement agencies also have experienced problems switching to such highly complex systems. In Alexandria, Assistant Police Chief Mark Canoyer said the police department put a new CAD system on hold because of "serious computer response delays."