RETURN Voice Networking

Dialing Plans and Phone Numbers

There are essentially two numbering plans used with the Public Switched Telephone Network; the North American Numbering Plan and the CCITT International Plan. Each of these is discussed in the following subsections:

North American Numbering Plan (NANP)

The North American Numbering Plan (NANP) consists of a 10-digit dialing plan that is broken down into two basic parts. The first three digits refer to the Numbering Plan Area (NPA). This is commonly referred to as the “area code”. The remaining seven digits are yet again divided into two parts. The first three numbers represent the Central Office code. The remaining four digits represent a station number.

Numbering Plan Area (NPA) Codes

NPA codes (area codes) are provided in the following format:

  N 0/1 X; where:  N is a value of 2 through 9
                   The second digit is either a 1 or a 0
                   The third digit is a value of 0 through 9


The second digit, when set to either a 1 or a 0 is used to immediately distinguish between 10 and 7 digit numbers. When the second and third digits are both 1, special action is taken. For example:

  411 = Directory assistance
  611 = Repair service (where implemented)
  811 = Business office (where implemented)
  911 = Emergency number

  211, 311, 511, and 711 numbers are reserved.


Additionally, the NPA codes also support Service Access Codes (SAC). These codes support 700, 800, 877, and 900 services.

Central Office Codes

The assignment of CO codes within a NPA is performed by the serving BOC (Bell Operating Company). Reserved for special use are:

  555 = Toll directory assistance
  844 = Time Service (where implemented)
  936 = Weather Service (where implemented)
  950 = Access to Interexchange Carriers under Feature Group "B" access
  958 = Plant test
  959 = Plant test
  976 = Information Delivery Service


Also reserved for use are some “NN0” (last digit “0”) codes.

Access Codes

Normally, a “1” is transmitted as the first digit to indicate a long-distance toll call. However, some special two digit prefix codes are used:

  00 = Interexchange Operator assistance
  01 = Used for IDDD (International Direct Distance Dialing)
  10 = Used as part of the 10XXX sequence.  XXX=specifies the equal-access
       Interexchange Carrier (IC).
  11 = Access code for Custom Calling services.  This is the same function
       as achieved by the DTMF "*" key.


The 10XXX sequence signifies a Carrier Access Code (CAC). The XXX is a three digit number assigned to the Carrier through Bellcore, such as:

  222 = MCI              223 = Cable & Wireless
  288 = AT&T             432 = Litel (LCI Intl.)
  333 = U.S. Sprint      234 = ACC Long Distance
  464, 555 = WilTel      488 = Metromedia Communications
  031 = ALC/Allnet


Feature Group “B” systems access the Carrier through 950-0XXX or 950-1XXX numbers. Feature Group “D” systems access the specified Carrier through 10XXX codes. Under Feature Group “D”, users have the option of specifying one IC as their primary carrier (presubscripion), therefore eliminating the requirement for the 10XXX access code.

Number Portability

At present, there are no interchangeable NPA (area code) codes, but it has been recommended by Bellcore that interchangeability be provided by mid-1995.

There are some CO codes that are interchangeable, and, allowing interchangibility at this level will increase the number of available codes by 152. However, once interchangeable CO codes are implemented within a NPA, the ability of the switching system to distinguish between 7-digit and 10-digit addresses becomes impaired. Thus some other possible method for making the distinction between 7 and 10 digit numbers is required. Some proposed methods:

- Dial "1" in front of all 10-digit calls (About 90% of BOC stations require
  this now).

- Implement a timeout (possibly four-seconds) after receipt of 7 digits to see
  if additional digits are input.

- A hybrid approach in which a four-second timer is implemented only when toll
  7-digit numbers are entered (1+NXX-XXXX).


At the time of this writing, a “hot” issue is that of “800” number portability. Presently, some customers can pay a lot of money to hold on to assigned 800 numbers, particularly those numbers that spell things; like 800-SKY-PAGE, 800-CALL-ATT, 800-PIN-DROP, etc. Should the customer change carriers new, and possibly, undesireable, phone numbers are assigned. This is an issue that can severly impact some businesses (e.g. mail order, subscriptions, etc.) and a planned date for resolution of this 800-Number portability issue has been set for May 3, 1993.

CCITT International Numbering Plan

In the early 1960s, the CCITT developed a numbering plan under which, the world is divided up into eight zones:

   1 = North America                     6 = South Pacific
   2 = Africa                            7 = USSR
   3 and 4 = Europe                      8 = Far East
   5 = Central and South America         9 = Middle East and South-East Asia


Additionally, each country is assigned a Country Code (CC) that can be either 1, 2, or 3 digits in length, beginning with the Zone digit.

Under the CCITT plan (Recommendation E.163), the international number consists of the Country Code, plus the National Number (NN). While CC can be 1-3 digits long, CC+NN cannot exceed 12 digits in length. Since international numbers consist of varying lengths, the local switching systems cannot always determine when dialing is complete. Therefore, a four-second timeout value is provided such that, if no digits are received within four seconds of the last digit, the number is considered complete and processed. To avoid this 4-second timeout, DTMF users can hit the “#” (octothorpe) key to terminate the timeout.