Notes- Telephone Numbers
During the next few yes, there have been several statements of the form, “It is conceivable that we may run out
of area codes and telephone numbers within the next 10 years.” The shortage of phone numbers is due to the
proliferation of cell phones, pagers and fax machines.
To simplify and manage telephone numbers, the North American Numbering Plan (NANP) was developed in 1947
and first implemented in 1951 by AT&T. NANP is an integrated telephone numbering plan of 24 countries and
territories: The United States and its territories, Canada, Bermuda, and 16 of the Caribbean countries. Although
the “North American” name of the calling plan, it does not include Mexico and Central American countries.
The current NANP number format can be summed up by the following table:
+1-NPA-NXX-xxxx
Number ranges
Component
+1
Name
ITU country
calling code
NPA
Numbering Plan
Area Code
Allowed range [2-9] for
the first digit and [0-8]
for the second, and [19] for the third digit
NXX
Central office
(exchange) code
Allowed range [2-9] for
the first digit and [0-9]
for both the second
and third digits
xxxx
Subscriber/station
code
[0-9] for each of the
four digits
Notes
Most often written without the
leading +1 generally indicated access
to long-distance service within NANP
Covers Canada, the United States,
parts of the Caribbean Sea, and some
Atlantic and Pacific Islands. The
area code is frequently enclosed in
parenthesis.
Often considered part of a
subscriber number. The three digit
Central Office codes are assigned to
a specific company serving its
customers, but may be physically
dispersed by redirection, or
forwarding to mobile operators and
other services.
This is the subscriber number or
station code.

Before 1955, the middle digit (P) of all area codes was limited to either 0 or 1. Just like today, the range
of the first digit (N) was [2-9], and the range of the third digit (A) was [1-9]. The area codes 800 and 911
were reserved for special purposes.

The restriction limit the middle digit, (P), was removed because all area coded had been assigned. Today,
the range of the first digit, (N) continues to be [2-9], the range of the second digit is [0-8], and the range
of the third digt, (A) is [0-9].

Area codes with the first two digits 37(A) and 96(A) are reserved for future use.

Area codes where P=A are called easily recognizable codes and are often assigned to special services such
as 800, and 877

Numbers 555-0100 to 555-0199 are reserved for fictional purposes, so they are discounted from the
actual possible number combinations.

(234) 234 5678 is valid, (123) 234-5678 is invalid since NPA cannot have a 1 in the (N) position
AQR-Telephone Numbers
1.
Name________________________Period___
Back in the day, the area code for Sugar Land was 713 and the only Central Office code was 494. How many
phone numbers were available to Sugar Land?
2. A second Central Office Code, 494, was added the existed prefix of 491. How many phone numbers were
available to Sugar Land?
3. The entire greater Houston area was originally assigned the area code of 713. How many phone numbers were
available to the area code prior to 1995?
4. How many phone numbers were available for the the United States and its territories, Canada, Bermuda, and 16
of the Caribbean countries after 1995?
5. As Houston grew, the original area code was restricted to phones inside Beltway 8, and the area codes of 281
and 409 were assigned to geographical areas outside Beltway 8. How many telephone numbers were available
after these new area codes were assigned?
6. If you were the commissioner of NANP, at the point where all the numbers were used up, what would be your
remedy to create more numbers?
7.
Some states are running out of license plate numbers. Delaware currently uses six-digit numbers in its
license plate numbering system, although recent reports show that its numbers are approaching 1 million
(Delaware uses retired numbers for new cars in many cases). The state of Washington recently stated that
it needs to explore options to its system of three numerical digits followed by three letters because it is
running out of numbers. New Jersey changed its system of three letters followed by two numerical digits
and one letter to a new system of one letter, two numerical digits, and three letters. (The last number
under the old system was ZZZ 99Z, followed by A10 AAA in the new system.)
a. How could you determine how many plate numbers New Jersey was able to assign
under the previous system? What assumptions did you make in your calculation?
b. How many additional license plate numbers can New Jersey assign under the new
numbering system?
c. Why do you think the first license plate under the new system was not A00 AAA?
d. How do New Jersey’s previous and new systems relate to the license plate numbering
systems used in Delaware and Washington?
e. What do license plate numbers tell you about the population in the state?
8. REFLECTION: In solving real-world problems such as estimating a crowd size, the number
of possible telephone numbers, or the answer to a Fermi question, why is it necessary to
first make some reasonable assumptions about the situation?