



Network Working Group                                        P. Robinson
Internet-Draft                                               Independent
Intended status: Informational                             14 April 2026
Expires: 16 October 2026


A Proposal for Long-Term Expansion of the North American Numbering Plan
                          (NANP) to 11 Digits
                    draft-robinson-nanp-expansion-00

Abstract

   The North American Numbering Plan (NANP) is projected to exhaust
   available telephone numbering resources within the coming decades
   under current allocation and utilization trends.  Existing mitigation
   strategies, including area code overlays and number pooling, extend
   the usable life of the NANP but introduce increasing operational
   complexity and user confusion.

   This document proposes a long-term, uniform expansion of NANP
   telephone numbers from 10 to 11 digits through extension of the area
   code or Numbering Plan Area (NPA) from 3 to 4 digits.  The proposal
   emphasizes backward compatibility, fixed-length numbering, and a
   multi-phase transition strategy designed to minimize disruption.
   This document is intended to stimulate discussion and does not
   represent the position of any standards body or regulatory authority.

Status of This Memo

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Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Design Goals  . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   7.  Proposed Expansion Model  . . . . . . . . . . . . . . . . . .   5
   8.  Routing Considerations  . . . . . . . . . . . . . . . . . . .   6
   9.  Transition Strategy . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Phase 0: Infrastructure Readiness . . . . . . . . . . . .   6
     9.2.  Phase 1: Dual-Format Acceptance . . . . . . . . . . . . .   7
     9.3.  Phase 2: User Notification  . . . . . . . . . . . . . . .   7
       9.3.1.  Segment 1 . . . . . . . . . . . . . . . . . . . . . .   7
       9.3.2.  Segment 2 . . . . . . . . . . . . . . . . . . . . . .   7
     9.4.  Phase 3: Mandatory Expansion  . . . . . . . . . . . . . .   7
     9.5.  Phase 4: Full Expansion . . . . . . . . . . . . . . . . .   7
   10. Alternatives Considered . . . . . . . . . . . . . . . . . . .   7
   11. Operational Considerations  . . . . . . . . . . . . . . . . .   8
   12. Economic Considerations . . . . . . . . . . . . . . . . . . .   8
   13. Security Considerations . . . . . . . . . . . . . . . . . . .   8
   14. International Considerations  . . . . . . . . . . . . . . . .   8
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   16. Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   17. Access to this document . . . . . . . . . . . . . . . . . . .   9
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     18.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     18.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The NANP currently utilizes a fixed-length 10-digit numbering format
   (NPA-NXX-XXXX).  Growth in telecommunications services, device
   proliferation, and number portability has steadily increased demand
   for numbering resources.

   Mitigation strategies such as overlays and thousands-block number
   pooling have delayed exhaustion but introduce increasing complexity
   in routing, administration, and user experience.




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   This document explores a uniform expansion of NANP numbers to 11
   digits as a long-term solution.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Definitions

   CP  Cellular Provider, the provider of service for a caller using
      cellular telephony.

   IXC  Inter-eXchange Carrier - the organization that carries a call
      between the caller's service provider and the called party's
      service provider where the caller's service provider does not
      serve the area of the called party.

   LEC  Local Exchange Carrier, or provider of service for a caller
      using a land line or VOIP service, the organization that provides
      dial tone and carries a call to the called party where they are
      within the service area of the LEC, or transfers the call to an
      IXC where they are not.

   N versus X  In the context of a telephone number, N is used to
      indicate a digit that is restricted to values of 2 through 9,
      while X indicates an unrestricted digit with the values 0 through
      9.

   NPA  The area code, or first three digits of the 10-digit telephone
      number.

   NPAX  The new area code, or first four digits of the new 11-digit
      telephone number.  This proposal recommends expansion of the NPA
      field by 1 digit and provides an expansion of the entire phone
      number to 11 digits.

   NXX  The prefix, or digits four through six of the 10-digit telephone
      number, or first three digits of the subscriber number.  This
      field is to remain unchanged, but is moved to digits five through
      seven of the new 11-digit telephone number.

   SIT tone  A Special Information Tone (SIT) is a standardized, three-
      beep audio signal (typically 950/1400/1800 Hz) played before a
      recorded announcement to indicate a telephone call has failed.



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   Subscriber number  The portion of the telephone number following the
      NPA or as this proposal recommends, NPAX.

   VOIP  Voice Over IP, or telephone service where the call initiates
      from or terminates via the Internet.

   XXXX  The last four digits of the subscriber number, or line number,
      digits seven through ten of the 10-digit telephone number.  This
      field is also to remain unchanged, but is moved to digits eight
      through eleven of the new 11-digit telephone number.

4.  Problem Statement

   The current NANP faces several challenges:

   *  Finite NPA capacity under existing numbering rules

   *  Fragmentation of numbering resources due to allocation practices

   *  Growing operational complexity in routing and database systems

   *  Long lead times required for major numbering plan changes

   A long-term solution should address these challenges while minimizing
   disruption to existing systems and users.

   All feasible approaches to expanding NANP numbering capacity
   introduce some degree of disruption.  The proposed expansion of the
   NPA is considered the least disruptive option, as it preserves the
   existing hierarchical structure of the numbering plan and minimizes
   changes to subscriber numbering and routing semantics.

5.  Design Goals

   The proposed solution is guided by the following goals:

   *  Maintain fixed-length numbering

   *  Minimize changes to existing routing logic

   *  Preserve compatibility with existing numbering structures

6.  Non-Goals

   The following approaches are explicitly not considered desirable:

   *  Variable-length telephone numbers




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   *  Region-specific numbering formats

   *  Frequent or repeated structural changes to the numbering plan

   *  Solutions requiring rapid or "flash cut" transitions

7.  Proposed Expansion Model

   This document proposes expanding NANP numbers from 10 to 11 digits by
   extending the NPA from three digits to four digits.

   Existing numbers:

     NPA-NXX-XXXX

   Expanded format:

     NPAX-NXX-XXXX

   During initial deployment, the fourth digit added to the NPA to form
   the NPAX MUST be selected such that it does not conflict with
   existing digit patterns used to identify the first digit of NXX
   codes.  Under current NANP rules, the first digit of an NXX is
   restricted to values 2 through 9.

   By selecting 0 or 1 for the additional NPA digit, the boundary
   between the expanded NPAX and the following NXX remains unambiguous.
   This allowsexisting digit analysis algorithms to distinguish between
   legacy 10-digit and expanded 11-digit numbers using a simple
   examination of the fourth digit, without requiring variable-length
   parsing or timing-based disambiguation.

   A single value (0 or 1) SHALL be used consistently across all NPAs
   during the initial deployment phase to ensure uniform behavior across
   networks.

   Example:

     213-555-1234  (legacy)

     2130-555-1234 (expanded)

   Or:

     303-555-1234  (legacy)

     3031-555-1234 (expanded)




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   This proposal preserves the semantic structure of the number

   *  NPAX is still geographic.

   *  NXX is still used as the routing block.

   *  XXXX is still the subscriber line number.

   The widespread adoption of overlay area codes has fundamentally
   altered the NANP environment.  A return to strictly geographic, non-
   overlapping area codes is no longer practical.  The proposed approach
   assumes the continued existence of overlays and does not attempt to
   reverse this trend.

   The designation of 988 as a nationwide service code required the
   elimination of 7-digit dialing in affected areas, accelerating the
   transition to uniform 10-digit dialing across the NANP.  As a result,
   this proposal does not impact legacy 7-digit dialing, as that
   capability has already been largely eliminated.

   This approach ensures that numbering expansion occurs at the highest
   level of the NANP hierarchy, avoiding disruption to lower-level
   components such as routing prefixes and subscriber numbers.

8.  Routing Considerations

   Existing routing systems rely on fixed field positions within the
   NANP number.  The proposed expansion preserves the relative position
   of the NXX and subscriber line number fields, allowing for minimal
   modification to routing logic.

   Systems that perform digit analysis MUST be updated to recognize the
   NPAX format.  This includes SS7-based switching systems, SIP routing
   platforms, and number portability databases.

9.  Transition Strategy

   A phased transition is recommended:

9.1.  Phase 0: Infrastructure Readiness

   Networks and systems are updated to support 11-digit numbers without
   public announcement.  Switching systems MUST be updated to recognize
   and correctly route NPAX-based numbers during Phase 0.







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9.2.  Phase 1: Dual-Format Acceptance

   Both 10-digit and 11-digit dialing are accepted.

9.3.  Phase 2: User Notification

   Phase 2 is implemented as two segments.

9.3.1.  Segment 1

   In Segment 1 of Phase 2, LECs, CPs, IXCs, and regulatory authorities
   MUST publicize the implementation of the expansion of the NPA to an
   NPAX, where the area code is expanded to four digits, and the
   telephone number to eleven digits.  An important highlight of the
   announcement SHOULD emphasize that there will be no change to the
   subscriber number.  It SHOULD also state the date Segment 2 will
   begin and the date that phase 3 will begin.

9.3.2.  Segment 2

   In Segment 2 of Phase 2, Intercept messages SHALL be imposed on
   callers dialing a 10-digit phone number, and such message SHALL
   inform callers dialing telephone numbers using the current 10-digit
   format of upcoming requirements.  The call SHALL still complete.  The
   message SHOULD state the date when dialing the new 11 digit number
   will be required.

9.4.  Phase 3: Mandatory Expansion

   11-digit dialing becomes required.  Callers dialing the old format
   10-digit number SHALL be presented with an intercept message
   beginning with a SIT tone and an announcement that they must dial the
   new 4-digit area code.  The message MAY announce the additional digit
   that MUST be dialed.  The call SHALL NOT complete, and SHALL be
   treated equivalently to dialing an invalid number.

9.5.  Phase 4: Full Expansion

   The fourth digit of the NPA is opened to all valid values, increasing
   numbering capacity.

10.  Alternatives Considered

   The following alternatives were evaluated:

   *  Further subdivision of number pooling blocks

   *  Expansion using only reserved NPA ranges (e.g., N9X)



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   *  Variable-length numbering schemes

   These approaches either provide limited long-term benefit or
   introduce undesirable complexity.

11.  Operational Considerations

   The proposed expansion is designed to minimize impact on:

   *  Call routing systems

   *  Number portability databases

   *  Inter-carrier signaling

   However, significant updates would be required in:

   *  Customer-facing systems

   *  Validation logic

   *  Legacy equipment and embedded systems

12.  Economic Considerations

   Incremental approaches distribute cost over time but increase long-
   term complexity.  A planned expansion incurs higher initial cost but
   may reduce cumulative cost and operational burden.

   Early planning enables gradual transition and reduces the risk of
   emergency implementation.

13.  Security Considerations

   Changes to numbering formats may impact fraud detection systems, call
   validation mechanisms, and authentication processes.  These impacts
   SHOULD be evaluated during implementation planning.

14.  International Considerations

   The proposed 11-digit format remains compatible with the E.164 [ITU]
   maximum length of 15 digits.  Coordination with international
   carriers and regulatory bodies would be required.

15.  IANA Considerations

   This document has no IANA actions.




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16.  Conclusion

   Expansion of the NANP to 11 digits represents a viable long-term
   solution to numbering exhaustion.  Early evaluation and planning are
   recommended to enable a controlled and gradual transition.

17.  Access to this document

   In addition to the IETF datatracker,
   https://datatracker.ietf.org/doc/recent, copies of all versions of
   this document are available from the author's Github repository, at
   https://github.com/electric-socket/11digitdialing.

18.  References

18.1.  Normative References

   [ITU]      International Telecommunications Union, "The international
              public telecommunication numbering plan Recommendation
              ITU-T E.164", November 2010, <https://www.itu.int/rec/
              dologin_pub.asp?lang=e&id=T-REC-E.164-201011-I!!PDF-
              E&type=items>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

18.2.  Informative References

   [RFC1394]  Robinson, P., "Relationship of Telex Answerback Codes to
              Internet Domains", RFC 1394, DOI 10.17487/RFC1394, January
              1993, <https://www.rfc-editor.org/rfc/rfc1394>.

Author's Address

   Paul Robinson
   Independent
   Email: comments@11digitdialing.com








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