Numbering index
USA numbering and dialling codes
The World Telephone Numbering Guide can be found at https://www.itu.int/oth/T0202.aspx?parent=T0202
Click here for a comprehensive discussion of code and subscriber number allocation.
Until recent years, the UK telephone system had two sets of dialling codes - National codes (earlier known as STD codes, STD standing for Subscriber Trunk Dialling) and Local codes. Since June 1995 there have been no local codes.
The UK was broken down into a number of areas known as Charge Groups, each having a main exchange that acted as the Group Switching Centre (GSC). Each of these Charge Groups was allocated a three-digit number preceded by the 0 (until 1995, when a 1 was added after the 0 for all geographic codes). The ‘0’ was the access digit from the local network into the trunk network. This breakout code (0) took you from the local network to a ‘register translator’ (R/T), which translated the following three digits into the code needed to route the call through the trunk network to reach the main exchange of the distant Group Switching Centre exchange.
The three digit codes were alphanumeric until the mid-1960s; for instance the code for Chester was 0CH4 (numerically 0244 and now 01244). Eventually usable alphanumeric codes were exhausted and all-figure codes were introduced.
Smaller exchanges were parented on the Group Switching Centres - usually dialling the digit 9 to access the GSC. The codes from the GSC to the dependent exchanges were usually two digits, in the range 81 to 80 and then additionally 71 to 70 if there were more than 10 routes from the GSC. Sometimes the codes could be single digits. Thus the STD or National code for a dependent exchange would be that for the GSC plus the local code from the GSC, followed by the subscriber’s number. The codes from one GSC to an adjacent GSC usually began with a 9, for instance 91, 92 and so on. These codes were barred to incoming trunk calls (you could not dial an STD code followed by a code starting 9), as the GSC should have been dialled directly using its own STD code, although in the early days of STD, when not all destinations were accessible by STD, this practice was feasible (even if not officially sanctioned).
Taking an example, the Welsh Tourist Board at Rhayader had the number (0597 82) 591 in which the 0597 was to reach the GSC at Llandrindod Wells, then a local code of 82 to reach Rhayader from Llandrindod Wells, followed by 591 - the subscriber’s number. Very often, telephone numbers were incorrectly set out. The recommended layout was ‘Rhayader (059782) 591’. The parentheses indicated 'optional', in other words Rhayader subscribers did not need to dial any code to reach numbers with the same STD code. Llandrindod Wells subscribers would dial 82 591 whilst other exchanges dependent on Llandrindod would dial 9 to reach Llandrindod followed by 82 591. (In some cases, subscribers might hear dial tone before dialling the last three digits; it was supposed to be suppressed on calls incoming to the exchange but this was not always carried out.)
As exchanges were converted to crossbar or electronic working, local numbers were converted to (usually) 6 digits. The first two digits indicated which exchange they were on and thus ‘linked numbering schemes’ came into being. Slowly all local codes within the ‘Charge Group’ disappeared, as the routing code was included in the 6-digit number.
In rural areas three-digit numbers indicated the existence either of UAXI3s (Unit Automatic Exchange No 13 - a pattern of small rural Strowger exchange dating from late 1930s to 1960s) or SAX's (Small Automatic Exchange, a reduced height rack - 8'6" - version of main Strowger exchange racks). The four-digit numbers were ‘Non-director’ Strowger exchanges. Any five-digit numbers were on TXK1-type Crossbar exchanges. Six-digit numbers were generally on TXE2 electronic exchanges, often configured as a linked numbering scheme).
An example could include Barmouth and Dolgellau, with six-digit numbers and TXE2 electronic exchanges. The nearby Dyffryn exchange was a UAX13 dependent on the GSC at Barmouth and accessed by dialling 7 from Barmouth or Dolgellau. In reality, Dolgellau subscribers picked up the telephone and were connected instantly to Barmouth. When they dialled 7, they were already connected to Barmouth and the 7 took then to Dyffryn. If the Dolgellau subscriber dialled a number beginning with 42, the link to Barmouth was dropped and the call connected within Dolgellau (all Dolgellau subscribers’ numbers begin with 42).
Alas, all the three digit numbers are but a memory now — in fact so is Dyffryn exchange; it is now a nice little holiday home! [Ian Jolly]
USA numbering and dialling codes
The following applies mainly to Bell System companies in the USA and Canada. Independent companies did not not always follow suit.
Bell exchanges tended, at one time, to reserve standard numbers as follows:-
1000 frame
1001 C.O. manager
1002 switchroom
1003 C.O. secretary (if there was one)
1004 lunchroom
1005 house service man
1006-1009 used by other managers (installation manager, etc.) or not assigned.
The US telephone system had two sets of dialling codes for services - the older series were used in rural areas, mainly on SXS (Strowger) exchanges. Some of these codes were also employed on British USAF bases.
112 Direct Distance Dialling (DDD) prefix in British Columbia and minor exchanges around Lincoln (Nebraska), still in use during the 1980s. Strowger only.
113 Directory Assistance operator. Strowger exchanges and after 1947, also on crossbar exchanges in predominantly Strowger areas.
115 Special long distance access code, generally for temporary use only.
116 Rural operator. Strowger exchanges.
117 Test Board.
118 Independent long distance company, used only in areas that formerly had both an independent and an AT&T local operating company.
118Tn Revertive calls to other parties on same line. T = wait for tone. n = 6, 7, 8, 9 to select desired party.
119n Revertive calls to other parties on same line.
110 Long Distance operator.
12 Direct Distance Dialling prefix (e.g. California)
Panel system and crossbar exchanges, also SXS exchanges in later years, use the following:-
1 followed by national number Direct Distance Dialling (DDD)
211 Long Distance operator (before DDD was introduced).
311 Automatic Number Identification (certain areas and ESS exchanges only).
411 Directory Assistance (equivalent of UK 192). 1921-1940 used on Panel exchanges only, then 1940-1963 panel and crossbar only, 1963-current, becoming universal.
511 Test board in some Panel exchanges.
611 Repair service (equivalent of UK 151). 1921-1940 Panel exchanges only; 1940-1960 also used in crossbar exchanges in predominantly Panel areas; 1960-current, used in most exchanges.
711 Test board
811 Business office (not implemented everywhere, equivalent of UK 150) or 811 nnnn to reach individual extensions.
911 Emergency (equivalent of UK 999 service)
0 Operator
Three-letter exchange names
Only four cities in the USA used three-letter prefixes; all others (and these four as well eventually) used two letters and a numeric digit.
- Boston, until 1948
- Chicago, until 1948
- New York, until December 1930
- Philadelphia, until July 1946.
New York was the only place to provide an overlap period (12 months), while both old and new codes worked.
According to Mark Cuccia, Los Angeles had mixed-length office codes, some being 2-digits (letters) only, some being 2-letters and one digit, thus the local numbering and dialing (until the late 1950s or early 60s timeframe) was mixed 2L-4N (6 'digit' local numbers) and 2L-5N (7 'digit' local numbers).
Reserved numbering blocks
Payphones tend to be numbered 8nnn or 9nnn. The blocks 99nn in New York and New Jersey and either 9nnn or 0nnn elsewhere are reserved for engineering purposes, for instance Loop Check and Milliwatt tone.
Local codes
Before the introduction of uniform seven-digit numbering within numbering plan areas (NPAs), single-digit dialling codes (e.g. 4, 5, 6) were used to reach neighbouring exchanges in some areas.
Service codes
Dial area code followed by
- 121 ... Inward assistance (equivalent to British 0.01 at AMC)
- 131 ... Information
- 141 ... Assistance (equivalent to British 01 at AMC)
- 151 ... equivalent of code 11
- 101 ... Engineers
These codes give service at parent auto-manual centre (AMC) of the numbering plan area (e.g. 212). If service is required at any individual exchange, dial area code followed by AMC code (-0XX) and service code. Example AMC codes:-
- 514-046 Bermuda
- 709-054 St Johns, Newfoundland
- 907-051 Anchorage, Alaska
- 907-05n Other places in Alaska
At gateway exchanges 11 is equivalent to code (12).
Gateway transits in North America
- San Francisco
- Miami
- White Plains, NY
- Montreal
- Vancouver.
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