Expanding the North American Numbering Plan

Fred R. Goldstein
ionary
11/99  (rev. 11/02)

Executive Summary

A method is shown by which the North American Numbering Plan, currently based on a 3+3+4 digit scheme created in 1947, is replaced by one with a greater capacity, more mnemonic number assignment, capacity for new services, and eventually 8-digit local dialing. This plan is implemented in six phases, with all changes having a "permissive" period and no "flag days" in which dialing plans suddenly become incompatible. Various "wish list" items that have been requested by the public and others are supported.

Definitions

• X - Any digit 0-9

• N - Any digit 2-9

• R - Any digit 2-8

• Y - The digits 1, 0

• NPA – Numbering Plan Area, or Area Code.

• Prefix – The 3-digit number identifying the rate center following an area code.

• SAC – Service Access Code (e.g., 800, 900)

• NXX – The current format for both NPA and prefix.

Goals and design basis

The North American Numbering Plan has been in place since 1947, and is based upon a well-known 3-3-4 digit numbering and dialing plan. The American public has grown accustomed to this plan and it is embedded in many places, mostly outside of the telephone network, where 10-digit fixed-length telephone numbers are stored in countless databases. But the rate of new area code assignment has been quite rapid, leading to the potential for exhaust in the 5-10 year time frame.

The Alliance for Telecommunications Industry Solutions (ATIS) has been working on numbering plan issues, but its efforts to date have been aimed mainly at creating an extension of the current plan, rather than migrating to a different one. ATIS appears to begin from the assumption that the new plan should retain as much as possible of the old plan, removing 7-digit local dialing in the process. While such a change is inevitable in the short term, an alternative is presented that leads to allows most calls to eventually be dialed with far fewer than the 11-12 digits required by the existing ATIS candidate plans, or especially the presently-favored 12-digit plan.

This proposed plan is inspired more by the new United Kingdom plan than by the North American plan that is nearing exhaust. In the UK plan, the first digit after the leading "0" (an escape digit roughly corresponding to the "1" in North America) indicates the type of code that follows, allowing the user to immediately recognize for instance "freephone", geographic, caller-pays wireless and premium-charge numbers. Unlike the UK plan, number length remains deterministic; the originating switch can determine the total number of digits to be dialed by analyzing the first few digits. This proposed plan leaves almost everything at a fixed 11 digits following an escape digit (which remains 1), while allowing local dialing to be accommodated with 8 digits, which should suffice to prevent splits and overlays in reasonably large areas, and even allows 7-digit local dialing in places that have a small enough number of prefix codes.

This plan is designed around its end state, from which a transition plan is created. This allows the focus to be on the long-term plan, rather than constraining the next NANP by compatibility with the existing plan. The transition plan phases in the new plan and phases out the old plan in a manner that avoids interruption. The worst case dialing scenario is a brief (e.g., one year) period in which 11 digits will need to be dialed for local calls; however, there will be so many overlays in place by then which require at least 10-digit dialing anyway that this will not be a major inconvenience. Existing 7-digit and 10-digit numbers are algorithmically converted to an 8-digit format, so no numbers are otherwise changed except by addition of a digit to the prefix. Like all other changes, this is initially implemented in a permissive mode.

End-state numbering plan

The plan allows digits 2-9 to be the first digits of an 8-digit (or, in some areas, 7-digit) local telephone number. The digit 0 retains its significance as an escape for special handling (international, operator, collect, etc.) and the digit 1 remains the escape digit for multiple other purposes. All calls using an area code (other than the old codes being replaced) must make use of the leading 1; the British custom of including it with the area code in written representations of the number is recommended. The end-state post-transition plan summarizes as per this table:
 

Note that the second digit after the leading 1 may not be "9" during the transition interval (see below). Thus for example the 12xx space does not include, at least initially, 129x.  Certain other restrictions and exceptions apply, as noted below.

Geographic USA numbers

The 12/13 space allows well over 100 new-plan area codes with 8-digit local dialing (most of 1200-1289; 1300-1369). These eventually replace, rather than supplement, the existing area codes. This allows many metropolitan areas to consolidate previous splits and overlays into a simplified dialing plan. Existing 10-digit geographic numbers (NPA-NXX-XXXX) will become 8-digit numbers by inserting a unique digit {0-4, 6-8} in the second position of the prefix (NRXX), with the R digit being mapped to which existing NPA the prefix code is in. Thus if current 617 and 978 are merged into 1267, then 617-234 may become 1267-2634 (the area code 1267 is not dialed for local calls) and 781-234 may merge into it as 1267-2734.

The selection of the second digit for the conversion of existing NXX codes is not always algorithmic, because existing NPAs to be consolidated may overlap in any of the three positions (e.g., 508 consolidates with 978 so the third digit is not appropriate there, but 310 consolidates with 323 so the first digit is not appropriate there). Nonetheless it is uniform within a region. The second digit "5" is not recommended because of potential conflicts with 950 carrier access codes.

It should be noted that prefix-pooling on the thousands-digit basis is retained, so that a given prefix code identifies a rate center, not a rate center and assigning local carrier. Thus 1267-2734-1xxx numbers may be assigned by one LEC and 1267-2734-2xxx numbers by another, if complete (unassigned) number pooling has not already been adopted.

The 137 space allows up to 100 area codes to retain 7-digit dialing. Relatively few places are likely to have few enough rate centers to only need 7-digit dialing, especially in the United States, but the option is retained. 138 is reserved for expansion of either the 7-digit or 8-digit space, as required.

Geographic non-USA numbers

The 14 space is used for non-USA areas. Canada might, for instance, be assigned 140-143 and choose to use 140x-142x for 8-digit local areas and 143xx for 7-digit local areas. Non-USA Caribbean points, billed at international rates, would then be assigned area codes in the 144-148 range, and could choose 7 or 8 digit dialing; with 7-digit dialing, the NPA would be one digit longer (148xx vs. 145x). It is assumed that most of these countries will retain 7-digit dialing.

Nongeographic numbers

The 15 space is used for nongeographic numbers, some with a total length of 11 digits after the 1. These include personal numbers like those once assigned in the 500 block, but can also be used for data services and other applications in which the caller pays a predictable usage-based rate. This might be local, corresponding for instance to the UK’s 0345 or existing American LATA-wide "oddball" numbers, or nonlocal; such services should probably be assigned different subsets of the 15 space. This numbering space may also be appropriate for data services that use E.164 numbers, such as B-ISDN/ATM/SMDS etc. The parsing of this space is not necessarily on a 3-4-4 or 4-3-4 basis, since it is never a home area. A subset of this space (such as 151-152) is reserved for short code dialing (see below).

Calling Party Pays

The 16 space is suggested for calling-party-pays services, with a total length of 11 digits following the 1. This is the norm for wireless in many countries; the number space is divided among wireless carriers and each carrier’s tariff to the caller is known ahead of time. Thus the caller knows the rate based on the prefix. For example, a carrier may be assigned the use of 16558 (plus 7 digits) for which it charges a rate of 15 cents per minute. This should overcome most objections to CPP services which, within the current NANP, could lead to "stealth billing" abuses. The 16 space is not necessarily reserved for wireless/CMRS, but that is the most obvious intended use.

Note: The demand for nongeographic (15) and CPP (16) numbers is speculative. An optional variation is to consolidate these two somewhat similar service classes behind a single space , reserving the other combination for future use. Thus nongeographic wireline numbers could be assigned 150 up and CPP numbers 158 down, leaving 16 for future expansion.

Carrier-specific numbering

The 17 space is used for carrier-specific purposes, which currently makes use of SAC 700. The total length of these numbers is 11 digits following the 1, as with most other services, but there is no need for specific parsing of this space on a 3-4-4 basis. The 178 space is specifically reserved for subscriber private numbering plans (access to ETNs, VPNs, etc.) and should not be used by carriers. (Thus a PBX need not for instance dial "8" for ETN and "9" for PSTN numbers.)

Toll-free

The 18 space is used for free-to-caller services, again with a total length of 11 digits following the 1. This allows a dramatic in the number of 800-like numbers. With judicious assignment, this space is sufficient to allow all PSTN subscribers to have their own toll-free number, if they choose to accept such calls.

Premium-rated services

The 19 space is used for premium-rated services, with a total length of 11 digits following the 1. This is akin to current 900 numbers, for which the charge may be unpredictable and which many subscribers might wish to block. Existing 7-digit premium intraLATA numbers (540, 976, etc.) should be consolidated into this space.

Restrictions

In all cases for number 12-19, the third dialed digit (second following the leading 1) must not be "9"; this is reserved for transitional purposes. Initially it will be used to accommodate old-style area codes; its future use is reserved for later expansion. Thus for example 12xx for geographic area codes includes 1200-1289, not 129x.

While special service codes of the form N11 do not require a leading 1, the pattern 1N11 is reserved to avoid confusion and to permit, for instance, 1411 to be used as an alias for 411, and to reduce false 911 calls. However the dialing pattern 1N00 is not reserved; current N00 service access codes are superceded.

Transition plan

Getting from the current 3-3-4 plan to this plan (basically 3-4-4 with optional 4-3-4 subsets) requires a number of steps. The following table illustrates them. The allowable formats for dialing (shown by X) are phased in and out in steps. Existing numbers are transitioned by using the reserved digit 9 in the second position of the NPA. Thus where n9xx is shown below, an example would be existing NPA 617 which becomes {1}6917. No new NPAs may have 9 in the second position.
 

Step A represents the first stage of transition, wherein the new-form transitional area codes become permissive with a leading 1 (for example, {1}617 becomes optionally 16917). This introduces no conflicts so it could be done at any time, but realistically will take a few years for supporting software changes to be made within the network. During this time 7-digit local dialing may still be permitted, but the widespread use of overlays will make it less common. Ten-digit or 11-digit local dialing and 11-digit (1-10) toll dialing is retained.

Step B is when 7-digit local dialing is finally disabled, to remove potential conflicts with new-format numbers before they are introduced. By doing this as a separate step, there is no "flag day" or any period of time when a given dialing pattern is ambiguous. Step B is suggested to occur roughly two years after Step A, allowing ample time for embedded systems to transition from 7-digit to 1x9xx+7d local (and toll) dialing. This could however be sped up, and if mandatory 10-digit local dialing is already in effect nationwide, this step becomes moot.

Step C introduces n9xx+7d transitional dialing for local numbers, without requiring a leading 1. This is strictly a short-term palliative to allow old-format local numbers to be manually dialed with 11 digits rather than 12, while 10-digit and 11-digit dialing of the old area codes is permitted but deprecated. Step C is suggested to occur a short time, up to say a year, after Step B. During the interval between steps C and D, 4-digit prefix codes are announced to supercede 3-digit codes, inserting a second digit as outlined above.

Step D marks the cutover of the new numbering plan, allowing an expansion in the available pool of numbers. At this time, 12-digit dialing (1nxx+8 digits) is introduced using newly-assigned area codes in front of 8-digit local numbers. 10-digit and 1+10 digit dialing of old-format numbers is disabled. Old area codes and numbers are still reachable using transitional n9xx+7 digit dialing, both with and without the leading 1, but the n9xx+7 format is on notice for discontinuance. Step D is suggested to occur roughly a year after Step C.

Step E marks the introduction of 8-digit local dialing, and the reintroduction of 7-digit dialing in smaller areas. The second digit of 8-digit local numbers may not be a 9, in order to remove a conflict with n9xx dialing of transitional 11-digit numbers. Areas retaining 7-digit local numbers (137xx+7) must disable n9xx dialing (in favor of 1n9xx) of local numbers before 7-digit dialing can be reinstituted. At this stage, autodialers and embedded applications are able to safely use the new 8-digit permanent numbers for local dialing, while manual dialing can use either transitional 1n9xx+7d or 8-digit local numbers. New numbers are assigned in 8-digit format. Step E is suggested to occur a year after Step D.

Step F marks the end of transitional dialing. The old 1n9xx area codes are disabled and that part of the numbering plan space is reclaimed for future use. Only 8-digit local (seven digits in some areas) and new-NPA dialing is permitted. This step is suggested to occur five years after Step E.

Additional features and special dialing sequences

The 11+ dialing space is reserved for its current application, feature code entry.

The 10+ dialing space is reserved for its current application, carrier access codes.

The 555 prefix in each area code is transitioned to 5955 while 5555 is also reserved as an alias.

The N11 dialing pattern for Special Service Codes (911, 411, etc.) is retained.

Feature Group B carrier access codes

The 950 pseudoprefix for Feature Group B carrier access codes is retained and expanded to 9n50, with the proviso that the n is not a 5. Thus 9250, 9350 et al are likewise reserved for access code use. These are not part of any area code, so a switch seeing an initial sequence of 950 or 9x50 will recognize a total access code length of 7 or 8 digits respectively. As a further restriction on the numbering plan, if 3-digit prefix codes are converted to 4-digit format by inserting a "5" in the second position, prefix codes 900-909 become ambiguous; old prefix conversions should thus avoid the use of 5 unless all 90x prefix codes can be removed from that area. After Step F, new prefix codes may be assigned 95xx provided that the xx is not 50.

Short codes

Short code dialing permits a nongeographic number to be less than the full length. Some subscribers use 950-series carrier access codes for this today; 555-series numbers in the old plan are likewise reserved. This is a natural function for the 15 space. Thus for instance short codes numbers can be assigned in the range 151 + 4 digits and 152 + 6 digits, while 150 is followed by 9 digits to make the usual 11-digit number (after the 1) length.