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Dialing Plan (16 May 1990)
1. Area Codes, North America
Bell Communications Research (Bellcore) is the organisation responsible
for the maintenance of the area codes and the dialing setup for
North America since the breakup of AT&T several years back. This
group sets standards for dialing, telephone number format, etc.
Area codes (or Numbering Plan Areas (NPA)) in North America are presently
of the form NZX where N represents a number from 2 to 8, Z is 0 or 1 and
X is any number. This distinguishes area codes from central office (exchange)
codes which are usually of the form NNX (ie. the difference between an
exchange and an area code is usually in the second digit, whether or not
it is a 0 or 1).
Area codes ending in -11 are not used, as they are normally used for special
services. The special purposes generally reserved are:
411 directory assistance
611 repair
811 business office
911 emergency
The other N11 codes are sometimes used within specific localities for
special purposes (711 is used for mobile service in British Columbia,
in the form 0+711)). In some cases, services are accessed through the
long distance network (ie. 1+411 or 1+611).
Area codes ending in -10 have been TWX numbers (teletype service).
TWX area codes include 510, 610, 710, 810 and 910. Normal telephone
numbers do not (yet) use area codes ending in -10, though these codes
will be reassigned to regular telephone service as TWX is eliminated.
Upcoming examples of this are the area code splits of 213 (Los Angeles
adds area code 310) and 415 (San Francisco adds area code 510).
Area codes ending in -00 are usually reserved for special services
like 700, 800 or 900. In fact, these codes are not considered to be
area codes as such, but Service Access Codes (SACs).
(2) Dialing Codes...
Theoretical capacity of the telephone system:
The original design of telephone numbers was:
(NZX) NNX XXXX
In theory, this gives:
N Z X N N X X X X X
8 * 2 * 10 * 8 * 8 * 10 * 10 * 10 * 10 * 10 = 1.024 billion numbers
However, as some area codes and exchanges are reserved for special
purposes (such as 411, 555 exchange, etc), the total possible number
of telephone numbers will be somewhat less.
As the exchange codes in some area codes were used up, some central offices
started using the NXX format, where the middle digit can then be a 0 or 1;
examples of these may be found in New York City or Los Angeles. The March
1990 conversion of the (416) area code around Toronto is another example,
as is a similar change to North Carolina's dialing around the same time.
The dialing is changed so that dialing 1+ a number always requires an area
code, and in the case of (416), will mean that 1+416+ will be needed for
long distance calls within (416). In New York City, however, all calls
within an area code are dialed with seven digits only (without dialing
1 first, let alone the area code).
Now, telephone numbers look like this:
(NZX) NXX XXXX
This gives a potential of:
N Z X N X X X X X X
8 * 2 * 10 * 8 * 10 * 10 * 10 * 10 * 10 * 10 = 1.28 billion numbers
However, codes like 411 and 611 would not be assigned because they will
still be needed for services such as directory assistance and repair.
Nevertheless, some unused N11 codes like 211 may be found in some area
codes as active exchanges. It's also not a good idea to assign the home
area code (could cause confusion).
Going from NNX exchange codes to NXX only represents a 25% increase in the
total theoretical amount of telephone numbers, and not all area code regions
are expected to run out of exchanges. Also, area codes do not generally go
outside a state or provincial boundary (the only exceptions are in Canada,
where 902 serves both Nova Scotia and Prince Edward Island, area 819 which
covers the eastern Northwest Territories as well as part of Quebec, and
area 403 which covers Alberta, Yukon and the western Northwest Territories).
The ultimate goal is not only to use area codes for exchanges codes, but
to use exchanges codes for area codes also. This means that telephone
numbers will ultimately look like this:
(NXX) NXX XXXX
This gives a potential of:
N X X N X X X X X X
8 * 10 * 10 * 8 * 10 * 10 * 10 * 10 * 10 * 10 = 6.4 billion numbers
With a five-fold increase in the number of possible area codes, there
should be plenty of room to grow for some time.
(3) Dialing Codes...
Basically, all calls within an area code will ultimately be dialed in
one of the following ways 1) dial seven digits, 2) dial 1 + home area
code + number, or 3) dial 1 + seven digit number within area code, then
wait for a few seconds to time out. One alternative not mentioned in
official documents (for touch tone phones) is to use 1 + seven digit
number in home area code + '#' with the # key terminating the dialing.
The initial set of new area codes will take the form NN0, or those numbers
ending in '0'. This means that areas codes that do not have exchanges
ending in '0' (or only a few NN0 exchanges that could be renumbered)
can still be able to tell the difference between an exchange and an
area code by looking at the first three digits.
The new NN0-type area codes will be assigned starting with these
first few codes (in order):
260, 480, 520, 590, 650, 220, 250, 490, 660, 680, 720, 730, 850, 940 ...
970 will be reserved for phone testing purposes, and is not slated to
become an area code.
All telephone systems are expected to change their systems over to allow
for the new style of area codes by 1 July 1995, or perhaps earlier. This
could mean that other area codes will be required to dial the area code
for long distance calls within area, even though the area code is not
running out of exchanges itself (eg. areas 519, 705).
Dialing Scheme