METHODOLOGICAL NOTE
A basic discussion of interchangeable area codes and how they effect the survey research industry. Includes a dissection of the telephone number as well as an examination of the rules governing the creation of an acceptable area code.
Within the survey research environment, telephone surveys remain the most popular method of data collection. Subsequently, constant monitoring of the telecommunications industry is a must in ensuring that the telephone samples we produce are efficient as well as statistically representative of the universe being surveyed.
The purpose of this methodological note is to examine the nature of telephone code assignments. Specifically, history on area code assignments and related impacts on telephone number assignments.
Telephone Number Dissection
In order to examine the process by which telephone codes are assigned, it seems appropriate to first examine the composition of the telephone number.
A telephone number consists of ten digits. For the purpose of this methodological note, we are going to divide the phone number into the following three components:
Area Code (AC). This section is, of course, the first three digits of a telephone number and is also referred to as the Numbering Plan Area (NPA).
Exchange (EXC). The exchange is made up of the next three digits. It is also referred to as the Central Office Code (COC), the Prefix or the NXX.
Suffix. This term refers to the last four digits.
Throughout the remainder of this document, we will also be using the following standard terminology:
7 Digits will be used to describe phone numbers consisting of only an exchange and a suffix.
10 Digits will describe a phone number that includes an area code along with the exchange and the suffix.
History
In 1947, the North American Numbering Plan (NANP) was introduced. Developed by AT&T and Bell Laboratories, the purpose of the NANP was to specify the format for telephone numbers, including the control and release of area codes and exchanges. Simply stated, the NANP decided when to activate a new area code or exchange.
When first introduced, the NANP controlled both area codes and exchanges. This remained in effect until the AT&T divestiture in 1980, at which time the controld of exchanges was transferred to the local Bell operating companies and other independant phone companies. Control of area codes was retained by the NANP administration.
Code Assignment Rules
This method is a compromise between the prefix and timing methods. The hybrid method requires timing only in situations where a toll call is dialed on a 1+7-digit basis AND the dialed exchange is currently active as both an area code and exchange.
In order to comply with phone switching equipment requirements, the NANP implemented certain rules as to specific 3-digit numbers could be used for an area code or an exchange.
Area Code Rules:
1) The middle digit of an area code could only be 0 or 1
2) Area codes could only begin with a 2,3,4,5,6,7,8 or 9 (not a 0 or 1).
3) Certain codes would be reserved for special services
--Codes ending with 00 (i.e., 800 or 900)
--Codes ending with 11 (i.e., 411 or 911)
If you start with 1000 possible three digit combinations (000-999), and then apply the rules, you end up with a total of 144 available area codes.
Exchange Rules:
1) The middle digit could only be 2,3,4,5,6,7,8 or 9 (not a 0 or 1)
2) Exchanges could only begin with 2,3,4,5,6,7,8 or 9 (not a 0 or 1)
3) Certain codes could be reserved for special services
--Codes ending with 11 (i.e., 411 or 911)
--Code 555
The most significant rules are ones pertaining to the assignment of the middle digit of each code. The end result of these rules was that each 3-digit number combination could only be an area code or exchange - not both. This allowed the phone switching equipment to differentiate between the two types of codes just by examining the first 3 digits of the number dialed.
Initially, the NANP assigned 86 area codes to cover the United States and Canada. In 1957, Alaska and Hawaii were added to the plan (Area codes 907 and 808 respectively). In 1958 area code 809 was assigned to incorporate Bermuda and many of the Caribbean Basin islands.
The NANP administration is also responsible for monitoring the need for new area codes, and projecting future needs. Projections are based on an annual survey conducted by the NANP administration, called the Central Office Utilization Code Survey (COCDS). Each year, central office code administrators (i.e. the local phone companies that are assigning exchanges and telephone numbers) provide an updated five year forecast of the demand for exchanges in each area code in their area. From this data the NANP administration projects both an exhaust date for each area code and a date on which relief will be required through the assignment of a new area code. NANP thus has a running view of the need for new area codes, and can project when the supply will be exhausted.
For details on how new area codes are introduced and implemented, please refer to GENESYS Methodological Note When the Area Code Splits.
Under the NANP plan, dialing procedures would be handled as follows:
Call Type Dialing Procedure
Local 7 Digits
Toll 1 (or 0) + 7 Digits*
Long Distance 1 (or 0) + 10 Digits*
*Dialing a 1 or 0 (before a toll/long distance call) indicates how the phone charges are to be billed (collect, credit card, bill to caller, etc...)
For the purpose of this methodological note, there is no difference between the 0/1 options. So, to make things less confusing for the remainder of this document, toll and long distance calls will be presented as 1+7 and 1+10 (even though the 1 can be a 0 or a 1).
The Problem
Under the 10-digit telephone number structure, there are a total of 10 billion possible numbers that can be assigned. However, factoring in the rules placed on the numbering convention (particularly the limitation on the area code's middle number having to be either a 1 or a 0), the pool of available numbers drops down to less than 1 billion. As with all finite sets, eventually the pool of available numbers will be exhausted.
At the time of its original design, the NANP was thought to have enough capacity to last until the turn of the century. However,by the late 1950's, it became apparent that new area codes were being assigned at a rate significantly higher than originally anticipated.
The Solution - The Interchangeable Code Plan
On the basis of COCUS results, the NANP administration notified the industry in January of 1992 that the solution to the problem -interchangeable area codes -would be needed in early 1995 and the industry must be prepared for this introduction on a full scale basis January 1, 1995.
The gist of the plan is to expand the capacity of the NANP by making area codes and exchange codes "interchangeable". This will allow codes formally reserved for exclusive use as area codes to be used as exchange codes and visa versa. Specifically, removing the "middle digit" restrictions implemented back in 1947.
One of the primary strengths of the interchangeable code plan is that it retains the current 10-digit number format, allowing for expansion of the available pool of numbers without requiring changes in currently assigned telephone numbers. As a result, the impact and cost of implementing the plan are minimized for the general public.
Under the traditional code plan, there were slightly less than 1 billion 10-digit numbers available. The interchangeable code plan will increase this available pool of numbers to more than six billion (enough to last us well into the next century).
When Will Interchangeable Codes Be Introduced?
Implementation of the interchangeable code plan actually began in February of 1974 when interchangeable exchanges were introduced in the 213 Los Angeles area code.
During 1993, the NANP administration assigned interchangeable area codes to go into service in early 1995 in Alabama, Washington and Arizona.
Full scale implementation of the plan is scheduled for January of 1995, at which time all industries must be prepared for the introduction of area codes that no longer have a zero or one as a middle digit.
How Will Interchangeable Codes Affect Us?
The rules defining the format of area codes and exchanges are not arbitrary. In using the codes (as defined by the rules), telephone switching equipment can distinguish between a toll number (1+7) and a long distance number (1+10). When these rules are suspended (through the introduction of interchangeable codes), the equipment loses the ability make that distinction.
In anticipation of interchangeable codes, some telephone operating companies have already upgraded their switching equipment to accommodate interchangeable codes. This will be further clarified at the end of this section.
Resolving the 7-digit vs. 10-digit Ambiguity
Three methods were proposed in order to rectify this problem:
The prefix method This method requires the caller to provide a positive identification of the length of the number being dialed. This would be accomplished by dialing a 1 before all 10-digit numbers. Under this method, there would no longer be a 1+7 digit option for toll calls. Toll calls, like long distance calls, would require the caller to first dial a 1 followed by the area code.
This plan is appealing in that major overhauling of telephone switching equipment would be avoided. Impacts on the general public would be minimal, as operator assistance messages would still be provided if the wrong dialing procedures were used. For the research industry (as well as any other industry or business with on-site telephone switching equipment) telephone software, hardware or both may need to be upgraded to accommodate dialing procedures.
The timing method This method requires the phone switching equipment to wait a period of time (approximately 4 seconds) after receiving the first 7 digits dialed (this is just on those numbers where the switch first receives a 1). If no additional digits are received, the switching equipment would then recognize it as a 7-digit call. If additional numbers are received, it would recognize it as a 10-digit call.
The advantages of this plan are that no changes to dialing procedures, and no modifications to on-site phone equipment. However, the distinct disadvantage of this method is that it adds to call/setup time and holding time for telephone equipment.
The hybrid method This method is a compromise between the prefix and timing methods. The hybrid method requires timing only in situations where a toll call is dialed on a 1+7-digit basis AND the dialed exchange is currently active as both an area code and exchange.
The method of choice is the prefix method. Under the prefix method, dialing of 1+7 digits for home toll calls is no longer permissible. Areas currently using the 1+ 7 digit dialing procedure to identify these calls will have to change their dialing procedures. Again, on-site switches/PBXs may need to be upgraded or if the technology is very old a new switch may have to be acquired.
So, What's the Bottom Line?
With the introduction of interchangeable codes, dialing procedures in your area may change, and require the prefix method of dialing. However, as stated earlier in this document, some areas of the country have already made the necessary upgrades to their switching equipment making the prefix method unnecessary. Details on these two groups are as follows:
You are calling from an area code who's switching equipment requires implementation of the prefix method in order to accommodate interchangeable codes.
Most of the country falls into this category (although this may change over the years as areas choose to make the upgrade to their equipment).
If your area falls into this category, dialing procedures are as follows:
Exchange Type Old Dialing Method New Dialing Method
Local, in your NPA 7 digits 7 digits
Toll, in your NPA 1+7 digits 1+10 digits
Long Distance 1+10 digits 1+10 digits
In many areas of the country, certain exchanges outside of the callers area code (i.e., in an adjacent area code) are also classified as local or toll. The old dialing procedures varied. New dialing procedures for these types of exchanges are as follows:
Exchange Type Old Dialing Method New Dialing Method
Local, outside your NPA Varied Varies, 7 digits or 1+10 digits
Toll, outside your NPA Varied 1+10 digits
2) You are calling from an area code who's switching equipment has already been upgraded to accommodate interchangeable codes.
As of June 1994, six states had upgraded their switching equipment (for all area codes) to accommodate interchangeable codes. These states were California, Illinois, New Hampshire, New Jersey, New York and Pennsylvania. As with Category #1, this list will change as phone switches currently unable to handle interchangeable codes are upgraded.
Exchange Type New Dialing Method
Local, In your NPA 7 Digits
Toll, In your NPA 7 Digits
Long Distance 1+10 Digits
Local, Outside your NPA 1+10 Digits
Toll, Outside your NPA 1+10 Digits
This Methodological Note has been written for general educational purposes only. For details about your specific area, information can be obtained from the following sources:
The Business Office of your Local Phone Company: This is clearly the best source of information about your specific area. They can provide the detailed list of exchanges (local and toll, within and outside of your area code). They can also provide the most up to date information on the dialing procedures.
Your Local Phone Book: In the front of your phone book is a listing of exchanges (local and toll, within and outside of your area code). However, remember that most phone books are only published once per year, so the information in your current directory may not address this issue. Also, since exchanges are continuously being activated in a given area, this process must be repeated annually (making your local phone company always the most current source of information).