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Frequently Asked Questions


Table of Contents

  1. What are CO codes?
  2. What are NPAs (area codes)?
  3. What is an area code "split"?
  4. What is an area code "overlay"?
  5. What is a "permissive dialing period"?
  6. What is "number pooling"?
  7. What is "boundary realignment"?
  8. What other types of telephony numbering services must be administered?

What are CO codes?

The term CO Code refers to digits D-E-F of the 10-digit NANP area address (where each digit is identified by an alphabetical character in the order ABC-DEF-GHIJ consisting of a 3-digit NPA (ABC), 3-digit CO Code (DEF) and 4-digit Line Number (GHIJ) in the format NXX-NXX-XXXX where: N = 2 to 9 and X = 0 to 9) (e.g., 740 is the CO Code in 613-740-1111).


What are NPAs (area codes)?

Numbering Plan Area, also called Area Code. An NPA is the 3-digit code that occupies the A, B, and C positions in the 10-digit NANP format that applies throughout the NANP serving area. NPAs are of the form NXX, where N represents the digits 2-9 and X represents any digit 0-9. In the NANP, NPAs are classified as either geographic or non-geographic.


What is an area code "split"?

The exhausting NPA is split into two geographic areas leaving the existing NPA code to serve, for example, an area with the highest customer density (in order to minimize number changes) and assigning a new NPA code to the remaining area.  This method divides areas by jurisdictional, natural or physical boundaries (counties, boroughs, cities, river, etc.) between the old and new NPAs.

This method has been the alternative chosen for practically all NPA relief situations prior to 1995.  NPA splits have occurred with enough frequency so that technical aspects have been addressed and established implementation procedures are generally understood.  Public education and acceptance of the process has been made easier because of the numerous NPA splits that have occurred.  This method generally provides long term relief for an area.


What is an area code "overlay"?

An NPA (or area code) overlay occurs when more than one NPA code serves the same geographic area.  In an NPA overlay, code relief is provided by opening up a new NPA code within the same geographic area as the NPA(s) requiring relief.  Numbers from this new NPA are assigned to new growth on a carrier neutral basis, i.e., first come, first served.   Mandatory customer number changes within the affected overlay relief area are eliminated.  In most cases, with the overlay relief method, 10-digit dialing is required for some of the affected customers' calling patterns.  Since the overlay relief method could result in unequal dialing for those customers served out of the overlay NPA, mandatory 10-digit dialing is recommended for all NPAs covered by the NPA coincident with the implementation of an overlay.

The overlay method reduces or eliminates the need for customer number changes like those required under the split and realignment methods.  It also allows the option to eliminate the permissive dialing period as part of implementation.  This method will necessitate ten digit dialing of local calls between the old and new NPAs as central office (NXX) codes are implemented in the new NPA.  NPAs have been previously implemented within an area and will vary with the individual characteristics of the area involved.  They are listed below:


What is a "permissive dialing period"?

The time frame beginning with the introduction of the new NPA whereby both the old and new NPA can be dialed. The beginning of permissive dialing is coincident with the relief date and ends with the mandatory dialing date.


What is "number pooling"?

Number pooling, in the context of current guideline, allows for sharing of central office codes (NXXs) among multiple service providers serving the same rate area.   All ten thousand numbers within each NXX code continue to be associated with the same rate area designation, but can be distributed among multiple service providers at the thousand block (NXX-X) level.  Examples of uses for blocks for which these guideline apply include plain old telephone service (POTS), Centrex, Direct Inward Dialing (DID), wireless service, facsimile, and coin phones.


What is "boundary realignment"?

In an NPA boundary realignment, the NPA requiring relief is adjacent to an NPA, within the same province, which has spare NXX code capacity. A boundary shift occurs so that spare codes in the adjacent NPA can be used in the NPA requiring relief. As a result, the geographic area of the exhausting NPA shrinks and the geographic area of the NPA with spare capacity expands. Only the customers in the geographic area between the old and new boundaries are directly affected the this change. This method applies to multi-NPA states or provinces only. It could provide for a better balance of central office (NXX) code utilization in the affected NPAs. This method is viewed as an interim measure because it tends to provide a shorter term relief than when providing a new NPA code.


What other types of telephony numbering services must be administered?

Other services include the personal communications services or 500-NXX codes, 900-NXX codes, the international inbound 456 codes, N11 codes (911, 411, etc.), 800-855-XXXX line numbers for the hearing impaired, carrier identification codes (CICs), and vertical service codes and automatic number identification information integers (ANI II).


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