covert@covert.DEC.COM (John R. Covert) (06/03/88)
How are prefixes assigned as an area code grows? Area Codes were assigned
nationally by evaluating the time it took to dial digits on a rotary phone
and assigning the most commonly dialed area "low numbered" digit combinations.
Are prefixes assigned the same way? A "time value" number could be
calculated as the sum of the three digits in the prefix (with zero counting
as ten).
The quickest to dial prefix (without N1X/N0X) is 221, with a time value
of 5 (2+2+1). The one that takes the longest to dial is 999 (value 27). With
N1X/N0X prefixes allowed, the shortest one is 211 (4) and the longest 900 (29).
Here in Virginia, the shortest to dial prefix, 221, serves Triangle, a
small town 40 miles south of Washington. Meanwhile, the busy Barcroft office,
serving parts of Arlington, Alexandria and Fairfax County, is stuck with 998,
among others.
Sometimes, things are done logically. At Dulles Airport, which always
had 661 as its only prefix since its inception, recently got its second
prefix, 260, a low number numerically, but with a higher time value (18) than
its predecessor (13).
I assume BellCore assigns prefixes, even to non-Bell operating companies
since there'd be bedlam otherwise. Do they discriminate and give prefixes
with zeros in them to non-Bell companies? In Northern Virginia, Contel has
22 prefixes in its service area, 7 of which end with a 0 (32%). If things
were truly random, only 10% of prefixes would end with a zero.
SPECIAL PREFIXES:
In most area codes, there are special prefixes set aside for either
internal phone company uses or for special services, some of which probably
haven't been invented yet:
976, 970: mass announcement
974: business office
954: repair service
958: reserved but unused most places, reads your number back in NY area
959: reserved but unused
950: slightly hacky long distance company access
There are also the high-volume prefixes for radio station contests and such,
but they're different in every area code. How did AT&T, which laid these
out before divestiture, know that there'd be specialized services and how many
there'd be so they'd know how many prefixes to set aside? Lucky guess?
There must be a lot of power to the set-aside. If an area code fills up,
it's split rather than using these prefixes for ordinary phone numbers.
Greg Monti, National Public Radio, Washington, DC +1 (202) 822-2459