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TUCoPS :: Phreaking Technical System Info :: xxxdial.htm

"D" Digit Release Another article on the possibility of turning the NXX prefix format into XXX, i.e. local numbers that start with 0 or 1
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Contribution #NANPE-189 



TITLE: Revisions to D-Digit Report 

DATE:  January 8, 2001

SOURCE: Penn Pfautz, AT&T


ABSTRACT: This contribution addresses an action item from the INC 53 NANPE workshop to propose revisions to the D-digit Report that, among other things, incorporate the  recent contributions. The text also notes open items that need to be resolved.


This contribution has been prepared to assist the INC Workshop.  This document is offered as a basis for discussion and is not a binding proposal on AT&T.  AT&T reserves the right to add to, amend or withdraw the statements contained herein.



INC Working Document

December 2000 

Draft D Digit Report 



  1. Background                  
  2. Identified Industry Uses of D Digit
  3. Implementation Issues
  4. International Concerns
  5. Requirements Documents
  6. Assessment Criteria
  7. Evaluation
  8.      Recommendation

Appendix A – Parking Lot

Appendix B - Contributions 

1. Background

The North American Numbering Plan (NANP) Expansion Workshop is actively working INC Issue 022, which was accepted on December 10, 1993.  Issue 022 involves the assessment of various options to expand the NANP from its current ten-digit format of NPA-NXX-XXXX.  An underlying assumption in these options is the release of the D digit.  The D digit is the first digit of a prefix or central office code; it is the fourth digit in the 10-digit format (NPA-NXX-XXXX).  The present designation of N denotes a number from 2-9.  Release of the D digit would permit any number from 0-9 in that position.  Should the D digit be released it could theoretically increase the quantity of available NANP numbers by up to 25%. Accordingly,  INC issue 159 was opened to investigate the technical impacts of D-digit release and come up with a recommendation as to whether or not the D-digit should be released. This report constitutes the resolution to issue 159. 

2. Identified Industry Uses of D Digit

In order to identify the impacts of D-digit release it is necessary to have a clear understanding of the myriad ways in which  NANP numbers with o or 1 as the D- digit are used in telecommunication networks. It is useful to distinguish two classes of use, intra- and inter-network. This distinction is important because, if the D-digit is released it will be up to individual carriers to determine how replace D-digit resources in intra- network use but the industry must agree on how to replace D-digit resources in inter-network applications.

2.1 Switch Applications


Examples of current uses of 0/1XX codes when used as the “D” digit are as follows:

  1. Test Codes

Editor’s Note: It’s not clear to me whether  the two bullets above consistent or redundant. One discusses use of prefixes with 10X codes; the other does not. I believe these codes can be used inter- as well as intranetwork but we should state the case clearly.

  1. Billing Numbers

These special billing numbers are recorded in AMA records and receive special treatment in downstream systems. In the case of OUTWATS they also control certain aspects of call processing that determine whether a given call can be placed. These number may be signaled between switches but are not passed between networks. 

Editor’s Note: Do we have an inconsistency? The first bullet says 01X is used for OUTWATS, the second 1XX. What are the alternatives but assignment of actual billing numbers to these customers?

  1. Pseudo Numbers

Editor’s Note: What are the alternatives but assignment of actual billing numbers to these customers?

  1. Feature Group D 0ZZ and 1NX Codes

0ZZ codes, used for trunk group selection in MF signaling, are drawn from spare Toll Center Codes, also GR-690-CORE, Section 

0ZZ and 1NX codes appear in the first stage of MF signaling for FGD inter-exchange calls.  Historically, the 0 and 1 were chosen as leading digits to avoid conflict with the 2-9 leading digits of both 7-digit and 10-digit telephone numbers.  In areas where D-digit release would be used, a restriction prohibiting the use of 7-digit numbers would be needed for MF FGD signaling. This is likely to be a significant issue only in LATAs that are single-NPA, since trunks would normally be arranged for 10-digit addresses in multi-NPA LATAs.

Editor’s Note: Should we consider these intra or inter network?

  1. Caribbean Codes

[0/1XX codes are used in the Caribbean for a variety of purposes including, USA direct dialing, international inbound 800, and some OSPS services.] to be clarified

Maintenance numbers

When dialing maintenance numbers to different area codes, the prefixes 012 and 013 can be used .

Editor’s Note: Can we elaborate on the purpose of these numbers and how they differ from test codes?

ESRD/ESRK E.911 Routing numbers

(per GR - 145 - CORE, Issue 2 May 1998)

Can we elaborate?

    1. Operator Services

These uses apply across networks.

2.2.1 Inward Routing


Today, operators can route to other operators for operator-to-operator contact for such things as inward BLV (Busy Line Verification), hard to  reach, etc., by dialing NPA-TTC-OSDC, where NPA is the area code (Number Plan Area), TTC is the Terminating Toll Center, and OSDC is the Operator Special Dialed Code. OSDCs are 3- to 5-digits in XXX - XXXXX format. The inward calls are routed on an NPA-TTC basis, if the NPA of the target operator system is different from the originating system. As with phone numbers, an operator would not have to dial the NPA if the operator were going inward to an operator system within their NPA. 

Toll Center Codes have the format 0/1XX, are preceded by the NPA, and are listed in Section 8 of the LERG.  (GR-1144-CORE, OSSGR: Section 6: Signaling, (Issue 1, March, 1997), Section 9.1 briefly describes the function of these codes, and Tables 13-3 and 13-6 in that document make it clear that routing to an OSS using Toll Center Codes populates the SS7 Called Party Number parameter – and thus must not conflict with end-user assignable NANP numbers.) A quick review of the Local Exchange Routing Guide (LERG, table 6ATC) shows that there are 415 active NPA-ATC combinations in service.  Of those codes, there are 93 different 0XX and 1XX values in use, ranging from 000 to 131. 22 of those 0/1XX values are used in only one NPA, while 32 of those values are used in 5 or more different NPAs. LERG Table 9ATC shows a total of 4278 defined routing patterns that are used for routing to those 415 ATCs. 

Operator services rely on the existing D digit restriction of a 0 or 1 to identify that the call is from another operator service. With this restriction lifted allowing NPA-0/1XX-XXXX calls to be routed from anywhere, there would be no simple method to determine that the call was from another operator. The switching systems use the D digit restriction to know that only another operator could route with a 0/1 in the D digit position. This restriction aids in preventing toll fraud since the receiving operator position does not bill the call. 

A prior condition for D-digit release would be re-arrangement of the ATC codes to occupy a smaller portion of the 0XX/1XX spectrum. Since ATC codes are used nationally, not just regionally, the requirement for an easily recognized and well-segregated portion of the number spectrum could be met only by re-arranging all ATCs, not just the ATCs in affected states or areas. Additionally, some NPAs currently have up to 7 defined ATCs, it would be prudent to allow a spectrum of more than 10 values, perhaps the range from 000 to 019.   

Equipment vendors  a variety of alternatives  to accommodate the TTC operator services in an environment when the D-digit had been released and only certain values were used for operator calls: 


2.2.2   Special Billing Numbers

Operator Service Systems query the LIDB (Line Information Data Base) to determine whether certain call types may be billed to given numbers. One type of LIDB record is named a Special Billing Number (SBN).  SBNs have the format NXX 0/1XX XXXX.  SBNs may be used for Calling Cards not associated with line numbers by LECs and non-LECs.  When offered by a LEC, the card may be known as an RAO card and with the first three digits identifying the RAO performing the billing.  When offered by a non-LEC, e.g., an Interexchange Carrier, the card may be known as a Call Issuer ID (CIID) card, with the first six digits of the card being called the CIID (said “sid”).  SBNs are referenced in several generic requirements documents, including: GR-1158-CORE, OSSGR Section 22.3:  Line Information Database, Issue 3, March, 1997 (GR-1149-CORE, OSSGR Section 10:  System Interfaces, Issue 2, March, 1997 and GR-1177-CORE, OSSGR: Special Billing Features, Issue 1, June 1997.)

Editor’s Note: What are the alternatives to provide equivalent services to customers?    Call Issuer I.D. (CIID)

Some time ago, it was decided that the way to provide IXCs (Interexchange Carriers) with TLN-based (Telephone Line Number-based) 10+4 calling cards was to allow ICs to assign subscriber card numbers that have the D digit as a 0/1. That is, the NPA-NXX-XXXX,, plus the four-digit personal identification number (PIN) -. After these numbers were made available to ICs, the LECs started using them for when more than one person needed a distinct calling card but had no distinct telephone number, e.g., multiple people using the same phone in an apartment but needing a separate calling card account. Since there were no phone numbers out there with a D digit of 0/1, there would be no collisions with any calling card assigned to a phone number. Now, if NXX becomes XXX, it is possible that someone might be assigned a phone number with the same NPA-0/1XX as somebody else's already assigned CIID card. This will cause a collision in the LIDB if both customers try to place a phone call and charge it to their card number. Will this break 10+4 calling card service? How would a Telco know which account should be billed? It is certainly possible that a Telco might not accept an IC's card, but what effect might a collision have in a clearing house, where all cards might have to be dealt with?


Editor’s Note: What are the alternatives to provide equivalent services to customers?

  1. Implementation Issues

It is not necessary to implement D-digit release nationwide and therefore, it could be accommodated on an as needed basis. However, all calling users throughout the NANP would need to have the ability to accommodate the dialing of a 0/1 in the 4th position in order to originate a call to an area that opened up the D-digit.  In most cases, if a foreign NPA were dialed, the originating switch would perform a 3-digit analysis on the NPA and route the call to the Inter-exchange network for further routing.In this instance the originating network would only need to translate the first three digits of the foreign NPA and route the call. (The only exception to this is if a foreign NPA is near or adjacent to the dialed NPA then the local switch may perform a 6-digit analysis). With the implementation of number portability, the Inter-exchange carrier would therefore, need to perform some type of analysis in order to properly route the call near the terminating network.The Inter-exchange carrier would therefore need to open up the 0/1XX CO codes in their database as assigned codes. 

D-digit release, in certain states or areas, would mean that subscribers in those states/areas would have to dial a full national number to complete any call. This dialing might be in form 1+ 10-Digits or 10-Digits with no prefix.  Local 7-digit dialing could be maintained in states/areas that did not open up the D-Digit within their own NPA(s).   

Impact on Future Number Expansion Plans

When, in the future, the NANP is expanded (for example, by expansion to 4-digit NPAs), the consideration above would not be in any way reduced. The use of 0 or 1 as the first digit of the central office code would continue to be subject to same problems and costs described in the previous paragraphs. Therefore, economic studies of cost-versus-benefit for D-digit release should not be limited to the current 10-digit NANP, but should also be applied to future plans.

Editor’s Note: Do we want to go here? Given   that we’ve narrowed our NANPE choices down to one that releases the D-digit, does this make sense? Instead, consider the following: 

The INC NANP Expansion workshop  has tentatively concluded that the most viable option for expansion requires that the D-digit not be released until after expansion since D-digit values of 0/1 will be used during permissive dialing to allow expanded format numbers to be distinguished from old format numbers. If the D-digit is released prior to expansion, a different expansion option, which the INC believes will be more disruptive of customer dialing habits and harder for customers to adjust to, would have to be chosen instead. Currently, approximately 10% of NANP resources are set aside for the possibility of that this other, less desirable option must be employed.

Coordination Across  NANP Countries

Consultations need to occur with the Regulatory Agencies of all nations within the NANP to determine if there is a particular use of the 0/1XX CO codes used in their countries. Preliminary investigation in the Local Exchange Routing Guide (LERG) only revealed codes that have been posted. However, it is assumed that other networks utilize these codes for internal network routing and potentially other purposes.

Customer Premise Equipment (CPE)

There may be an issue with certain types of CPE. This equipment may have been set up with some logic that would identify the call as invalid if the D-digit is dialed with either a 0 or a 1. The magnitude of this issue can not be determined but with the other significant numbering changes, e.g., INPA, a percentage of the CPE equipment could not accommodate the opening up of the B-digit. Therefore, it could be assumed that a certain percentage of the CPE equipment may not be able to accommodate the opening up of the D-digit. 

3. International Concerns Outside the NANP


There are no known international concerns with respect to the D digit release since foreign carriers do not typically analyze calls down to the central office code level.  There are also no restrictions on the values of the digits within the 15-digit E.164 number. 

6. Industry Requirements Documents


Local switch NPA+0/1 dialing restriction  

Many requirement documents include a reference to the format of 10-digit NANP numbers that are assignable to end users as NPA NXX-XXXX (with the NPA also having an NXX format).  Particular examples are:  

  1. Assessment Criteria

In assessing whether the D-digit should be released, it is necessary to consider


Appendix A – Parking Lot


Appendix B – Contributions

This contribution, provided by Nortel Networks, lists existing published uses of a “0” of “1” as the first digit. 

NANP NETWORK (AT&T / BOC-LEC / Stentor-Canada / C&W-Caribbean / etc.)

ROUTING CODES  0XX / 1XX / special NXX

Includes historical use of some codes as well as many current generic uses

Code - Description

000 - Rate Quote System

001 -> 005 - (spare- available for: TTC's, pseudo c.o. codes, etc.)

006 - Intl. ATME (TAS-2 Domain ONLY) 006 ->

008 - InWATS OSO routings (from multiple states/NPA's) (1960's/70's)

009 - Rate Quote System

010 - (RESERVED)

011 - IDDD Access / IOTC: International Originating Toll Center

012 - Alt.Route via Principal Tandem (i.e., due to Sector Tandem failure)

013 - (TWX function - but what?)

014 - 4-Row TWX Assistance Opr (954-1212 converts to 014-1212): from 510 TWX (1960's/70's), and Canadian TWX Operator from certain provinces (1960's/70's/80's) sometimes from 710/810/910 TWX (1960's/70's),

015 -> 019 - TWX: NPA/510/610 to N10 routings, N10 to 510/610 routings (note that the N10 SAC is converted to 01N)(1960's/70's and Canada 1980's)

020 -> 021 - TWX: N10 (4-R) to NPA (3-R) routings (note 0+NPA) (1960's/70's)

022 -> 029 - (spare- available for: TTC's, pseudo c.o. codes, etc.)

030 -> 031 - TWX: N10 (4-R) to NPA (3-R) routings (note 0+NPA) (1960's/70's)

032 -> 039 - (spare- available for: TTC's, pseudo c.o. codes, etc.)

040 -> 041 - TWX: N10 (4-R) to NPA (3-R) routings (note 0+NPA) (1960's/70's)

042 -> 049 - (spare- available for: TTC's, pseudo c.o. codes, etc.)

050 -> 051 - TWX: N10 (4-R) to NPA (3-R) routings (note 0+NPA) (1960's/70's)

052 -> 059 - (spare- available for: TTC's, pseudo c.o. codes, etc.)

060 -> 061 - TWX: N10 (4-R) to NPA (3-R) routings (note 0+NPA) (1960's/70's)

062 -> 069 - (spare- available for: TTC's, pseudo c.o. codes, etc.)

070 -> 071 - TWX: N10 (4-R) to NPA (3-R) routings (note 0+NPA) (1960's/70's)

072 -> 079 - (RESERVED- for Autovon?)

080 -> 081 - TWX: N10 (4-R) to NPA (3-R) routings (note 0+NPA) (1960's/70's)

082->087,9 - InWATS Tndm routings (third-digit Band; 089 Bnd-1)(1960's/70's)

088 - (spare- available for: TTC's, pseudo c.o. codes, etc.)

090 -> 091 -TWX: N10 (4-R) to NPA (3-R) routings (note 0+NPA) (1960's/70's)

092 -> 099 - (spare- available for: TTC's, pseudo c.o. codes, etc.)

100 - Plant Test: Test Board

101 - Plant Test: Test Board

102 - Plant Test: Milliwatt Tone (~1004 Hz)

103 - Plant Test: Signaling Test Termination

104 - Plant Test: 2-Way Transmission and Noise Test

105 - Plant Test:Auto.Transm.Measurement System/Remote Offc.Test Line

106 - Plant Test: CCSA Loop Transmission Test

107 - Plant Test: Par-Meter Generator

108 - Plant Test: CCSA Loop Echo Support Maintenance

109 - Plant Test: Echo Canceler Test Line

110 -> 119 - Operator Codes

11362 - Air-to-Ground / Maritime-Marine / High-Seas / Trains

115X(1) - Leave Word

116X(1) - Calling-Card Validation System

120 - Network Emergency Center

121 - Inward Operator

122->127,9 - InWATS TSO routings (third-digit Band; 129 Band-1)(1960's/70's)

122 - AT&T Ready-Line 800 (1980's)

128 - (RESERVED)

130 - TWX Assistance Operator (3-Row) (1960's/70's)

131 - Directory Operator

132->137,9 - InWATS TSO routings (third-digit Band; 139 Band-1)(1960's/70's)

138 - Mexico: Hermosillo SON- Numbers routings (pre-1977); IDDD Equal Access routings- 011+ (mid-1980's on)

140 - TWX Assistance Operator (4-Row) (1960's/70's and Canada 1980's)

114 - Rate & Route Operator

142->147,9 - InWATS TSO routings (third-digit Band; 149 Band-1)(1960's/70's)

148 - Mexico: Hermosillo SON- Operators (1970's, 1980's)

150 - Cable Control: Hawaii (WAS 157 until 1977, when PR got 800-468)

151 - International Assistance Operator

152->157,9 - InWATS TSO routings (third-digit Band; 159 Band-1)(1960's/70's)

158 - Mexico: Chihuahua CHI- Numbers routings (pre-1977); IDDD Equal Access routings- 01+ (mid-1980's on)

160 - International Operator Center (IOC)

161 - Trunk Trouble Reporting

162->167,9 - InWATS TSO routings (third-digit Band; 169 Band-1)(1960's/70's)

168 - Mexico: Chihuahua CHI- Operators (1970's, 1980's)

170 - Mexico: Monterrey NL- Numbers routings (pre-1977)

171 - Mexico: Monterrey NL- Operators (1970's, 1980's)

172 - Caribbean: Dom.Rep., Puerto Rico, US Virgin Islands (1970's)

173 - Caribbean: Bahamas (1970's)

174 - Cable Control: Puerto Rico, US Virgin Islands (1970's)

175 - Caribbean: Jamaica (Cayman Islands) (1970's)

176 - Caribbean: Bermuda (1970's); Mexico: Mexicali BCN- Operators (1980's)

177 - Cable Control: Dominican Republic (1970's)

178 - Caribbean: Barbados (Dominica, St.Lucia, St.Vincent) (1970's)

179 - Caribbean: Trinidad/Tobago (Grenada thru 1986) (1970's)

180 - Mexico: Mexico City DF- Numbers routings (pre-1977); Numbers routings for _ALL_ of +52 Mexico (1977-on)

181 - Toll Station Operator (for Ring-Downs)

182 -> 188 - ISCs- International Switching Centers (Gateways):

182 - WHPLNY: 02 0201T 4A (70s); 104T 05 0504T 4E (80s); 103T 03 0203T (90s)

183 - NYCMNY: 04 AA02T 4M (70s); 063T 24 BW24T 4E (80s); 111T 55 BW55T (90s)

184  - PITBPA: 02 DG42T 4A (70s); 076T 03 DG43T 4E (80s); 115T 09 DG09T (90s)

185  - JCVLFL: 02 CL01T 4A (70s); 041T 03 CL03T 4E (80s); ATLNGA: 007T 04 TL01T (90s); VANCBC: 02 0104T 4A (70s)- CANADA

186 - OKLDCA: 03 0341T 4M (70s); 070T 06 0344T 4E (80s); SCRMCA: 083T 04 0404T (90s)

187 - DNVRCA: 03 MA03T 4A (70s); 027T 05 ZJ05T 4E (80s); SHOKCA: 084T 05 0296T (90s)

188 - NYCMNY: 10 BW01T 4A (70s); 062T 50 5450T 4E (80s);?013T -- 5410T (90s)? MTRLPQ: 01 0201T 4A (70s)- CANADA

189 - Mexico: Mexico City DF- Operators (1970's, 1980's)

190 - Mexico: Operator routings for _ALL_ of +52 Mexico (1980's)

191 - Conference Operator Loop-Around (1970's); AT&T Advanced 800 Intercept Recording Frames (1980's)

192 - Cable Control: Jamaica (Cayman Islands) (1970's)

193 - Cable Control: Trinidad/Tobago (Grenada/Carriacou) (1970's)

194 - Cable Control: ?? Bahamas ?? (1970's); Mexico: Tijuana BCN- Operators (1980's)

195 - Cable Control: Antigua/Barbuda (Anguilla, Br. Virgin Islands, Montserrat, St.Kitts/Nevis) (1970's); AT&T Advanced 800 (1980's)

196 - TTC for Oprs/Tst at Wayne PA RC (1960's) as: 196+1X1/11XX/10X  [Wayne was not Principal City for NPA 215 (Philadelphia-2 was), but circa 1972 Wayne got its own 215+0XX+ Oprs/Tst TTC Code]; Cable Control: Barbados (Dominica, St.Lucia, St.Vincent) (1970's); AT&T International 800 (1980's)

197 - Caribbean: Antigua/Barbuda (Anguilla, Br. Virgin Islands, Montserrat, St.Kitts/Nevis) (1970's); AT&T Direct-Services Dialing Concept (1980's)

198 - TTC for Oprs/Tst for Rockdale GA RC (1960's): 198+1X1/11XX/10X [Rockdale was not Principal City for NPA 404 (Atlanta was), but circa 1972 Rockdale got its own 404+0XX+ Oprs/Tst TTC Code]; Cable Control: ?? Bermuda ?? (1970's); AT&T International City Service Center (ICSC) (1980's)

199 -Mexico: Monterrey NL (Oprs? Numbers?) (1960's); Cable Control: Alaska (1970's); AT&T USA Direct (1980's)

11XX(X) - Operator Codes (Leave Word, BLV/Interrupt, Card Validation):

11362 - Mobile / Marine-Maritime / High-Seas / Air-Ground / Trains

1150(1) - Universal or Coin Callback Operator

1151(1) - Conference Operator

1152(1), 1182(1) - Mobile Service / Air-Ground Operator

1153(1), 1183(1) - Marine Service Operator

1154(1), 1188(1) - Toll Terminal Operator

1155(1) - Time and Charges Callback Operator

1156(1) - Hotel / Motel Callback Operator

1157(1) - IOTC Access Trunk

1158(1) - BOC/LEC Inward Completion Assistance

1159(1) - BOC/LEC Inward Busy-Line Verification

1160(1) - Calling Card Validation (Dial-Pulse)

1161(1) - Calling Card Validation (DTMF)

1162(1) - Calling Card Validation (MF) 

OLD 3-digit "Step" Service Codes (generic uses; both customer and operator) 

112        DDD Toll/Tandem/CAMA Switch Access

113        Directory Assistance (Information)

114        Repair Service

115        Mobile/Marine/Air-Ground/Conference Operator

116        Local Area Toll Station Operator

117        Test Board

118-N-1    Ringback (Multi 4/8/10/Rural Party Lines)

119-1      Ringback (Two-Party Lines)

110        Outward Toll Cordboard Operator 

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