By Dec. 15, residents in Arlington and Fairfax counties and Alexandria will be able to dial 911 to call police, firefighters or an ambulance, according to planners.

The emergency system, originally planned for a July 1 start-up, was delayed because local jurisdictions wanted a more sophisticated system than that originally proposed by the C&P Telephone Co., according to Thomas Brannon, spokesman for the Northern Virginia Planning District Commission, which has been coordinating the 911 project.

Used in cities across the country, the 911 system uses a phone number that is easy to remember and provides almost instant communication with police and fire officials. The system is already being used in Prince William and Loudoun counties for fire and rescue services, although it is not used to call the police or sheriff, Brannon said.

The system here will permit instant transfer of calls from one jurisdiction to another in the many Northern Virginia neighborhoods that straddle boundary lines.

At Bailey's Crossroads, for instance, where Fairfax, Arlington and Alexandria meet, telephone exchanges bear no relation to the geographical boundaries and police need to be able to push a button and immediately transfer calls to the proper place, Brannon said.

The first-year installation and operating cost for the system will be $150,000; in subsequent years, the operating cost will be $143,000. The costs will be divided among local jurisdictions.