The 911 emergency telephone system should be operating in most of Northern Virginia by next July 1, under plans approved this week by seven area jurisdictions.

Residents of suburban Maryland, the District, Prince William County and many other metropolitan areas around the country already can summon police, fire or ambulance assistance by dialing 911.

Northern Virginia governments have been considering a regional 911 system since 1971, but couldn't get a majority to approve it until this fall when Alexandria, the last major holdout, agreed. That city had been holding out for a quicker and more expensive computer-automated system, which the C&P Telephone Co. expects to be able to install in 1982.

A regional system is the only feasible one for Northern Virginia, where telephone districts cross political boundaries.

Only the towns of Herndon and Vienna have declined to join the 911 system, in which calls are switched manually, on hot lines, to appropriate police, fire or ambulance numbers.

A spokesman for the Northern Virginia Planning District Commission, which has helped organize the 911 system here, said calling the emergency three-digit number in most cases would be faster than dialing conventional seven-digit emergency numbers, which few persons have memorized and many would have to look up or get from an operator. The 911 system also avoids confusion over which jurisdiction to call for help, since thousands of Fairfax County residents have Alexandria or Arlington mailing addresses or phone numbers.

As an example, a planning district commission spokesman said, the telephone company's Barcroft station, one of 23 main telephone stations in Northern Virginia, includes nearly 35,000 phones for homes and businesses. Of those, more than 12,000 are in Alexandria, 13,000 are in Arlington and 9,000 are in Fairfax. Operators answering the 911 number in that district would learn the first three digits of the caller's number and switch the call to the right jurisdiction.