By the fifties, the major cities of Birmingham, Edinburgh, Glasgow, Liverpool, London and Manchester each had a unified numbering scheme, so no matter whereabouts within that city any customer on an automatic exchange could dial any other by dialling the 3-letter exchange name code and a four digit number. The equipment required to make this work was called a director, hence these cities are known as director areas.
In the rest of the country things were far less simple, due to the absence of a director they were known as Non-Director (ND) areas. Before the arrival of Subscriber Trunk Dialling (STD) in 1958 an ad-hoc string of digits were required to access one exchange from another. Calls were charged by the distance between calling and called exchanges, up to 5 miles: 1 metered unit, 5 to 7.5 miles: 2 metered units, 7.5 to 12.5 miles: 3 metered units and 12.5 to 15 miles: 4 metered units. Calls were untimed, the metered units were applied when the caller answered, then the parties could talk for evermore. Charging by radial distance meant that every exchange had a unique charging regime.
With the advent of STD there was a need to simplify charging, clusters of exchanges were allocated to a Charge Group and charging was based on the radial distance between the centre of the clusters. A local call was designated as a call within your own Charge Group and to the adjacent Charge Group's exchanges. Call charging took into account both call duration and the time of the day the call was made.
STD brought about a standardised numbering scheme for the whole of the country, not only for long distance calls, but local codes between exchanges too. Each charge group was allocated one or more STD area codes. The original format was Zero-two letters-a number followed by the 4 to 6 digit telephone number. In March 1966, All Figure Numbers (AFN) replaced the old letter codes in both the big city director areas and STD codes too. For example Kettering 0KE6 became 0536 and Northampton 0NO4 to 0604.
Local codes were standardised too. Codes between the parent Group Switching Centre (GSC) and its local exchanges were usually one, two or three digit numbers starting with 6, 7 or 8. Codes to adjacent charge groups, whether small exchanges parented on this GSC or to other GSCs would be two or three digit numbers starting with a 9. The reasoning will be explained later.
It wasn't until 16 April 1995, that the current nation numbering scheme came into being, in what was known as 'phONEday' when a national number change took place introducing the digit 1 between the Zero and the first code digit. The afore mentioned codes 0536 and 0604 became 01536 and 01604 respectively.
The original nineteen fifties London code of '01' had earlier been split into Inner London 071 and Outer London 081 on 6 May 1990. On phONEday these became 0171 and 0181. The current London code 020 was introduced in a further mass code change on 1 June 1999, then codes 0171 xxx xxxx and 0181 xxx xxxx became 020 7xxx xxxx and 020 8xxx xxxx respectively. Had Oftel (now OfCom) been better at planning, this later mass change could have been executed in 1995.
My interest in call routing was reflected in my apprenticeship notebook, in 1971 drawing a diagram showing every local exchange interconnection and the dialling code. Figure 1, reproduces the sketch and may be the only such diagram published showing the whole of a Group Switching Centre (GSC) area.
The GSC at Kettering is the parent exchange for its own customers and for ten exchanges in the Kettering 0536 charge group, six in the Wellingborough 0933 charge group and three exchanges in the Clopton 0801 charge group. Kettering's connection to these exchanges are shown by a black line on the drawing. For most local calls, the remote exchange dials a 9 into Kettering and then out to the destination. The Auto-Manual Centre (AMC) is collocated with the GSC, so dialling 999 takes the caller into Kettering for the emergency operator.
The small exchanges surrounding Corby and Wellingborough were provided with sideways routes as they have a community of interest with the major town. These are coloured blue when they are within the same charge group. Wellingborough has a wider community of interest and was provided with routes outside its charge group area, shown as green lines. Further sideways routes existed in the Oundle and Thrapston areas.
Where sideways routes were provided, these would be used in preference to dialling through the GSC. For example a Oundle customer calling Bythorn would dial 785 so the call routed Oundle → Thrapston → Bythorn. The Rockingham → Brigstock Code was 874 not 973, routing via Corby and not the parent at Kettering. This means that every exchange in any one GSC area has a unique set of dialling codes. Customers who move between towns in their daily life could easily forget where they are and dial the wrong code, resulting in a completely wrong number and frustration for both parties.
Abusing the System
A local call rate is charged for calls within one's own Charge Group and to exchanges in adjoining charge groups. Beyond that, calls were charged at one of three STD rates based on radial distance. Wellingborough (0933) and Clopton (0801) Charge Group's exchanges are charged 'A' rate for calling Market Harborough and its seven small dependant exchanges. Those exchanges with STD facilities could dial the call themselves, but those without would have to dial the operator, who would charge them a higher 'A' rate than for automatically connected calls. A similar situation applies for Oakham and its seven exchanges in the 0572 charge group.
Customers in the 'know' in the ten exchanges in the Wellingborough (0933) and Clopton (0801) Charge Groups could dial the unadvertised code 996 for the Market Harborough or 997 for the Oakham and their dependent exchanges and get their call at local rate. There was great reliance on alternative codes not being widely known to the public. The official code for Oundle 0832 Charge Group exchanges making a local call to the Huntingdon 0480 Charge Group which they bordered, routed via the parent exchange at Peterborough, then dialling a secret code for Huntingdon. The advertised dialling instructions at Peterborough for Huntingdon was to dial the 'A' rate STD code 0480, but if they dialled 98 it would be a local call. The network could be exploited for calls further afield by through dialling, chaining together published codes, which is dealt with later.
Incoming STD Calls
STD codes in the seventies, could be between 8 and 10 digits long, in non-director areas this would be 4 digits for the charge group code and 4 to 6 digits for the access code and number. Calls from the rest of the country to Kettering 0536 'home' charge group are translated to route through the trunk network to Kettering GSC.
For example calling Burton Latimer 2190, the number would have ten digits 053672 plus 2190, or calling Kettering 4201, has only eight digits 0536 then 4201. Examining Figure 1, it would seem possible to call Bozeat 299, by dialling 0536 then 924 and 299 as this falls within the 10 digit limit. However the outgoing STD equipment at the caller's GSC does not permit 1, 9 or 0 to be dialled immediately following the STD code. This was the reason for using codes beginning with a 9 for other charge groups.
Wellingborough 0933 is a dependent charge group with no GSC of its own. Distant exchanges translate it to Kettering 95. The dialling code for Wellingborough town was 09333, the extra 3 on the end is used to make up the routing Kettering 953 and a maximum of a 5 digit Wellingborough number. The STD code for Rushden was 09334, for example Rushden 3010 translates to Kettering 954 plus 3010 the call routes from Kettering to Rushden and not via Wellingborough. The use of a charge group name does not imply the call routes through that exchange when its not a GSC in its own right.
There is an added complication, Bozeat, Finedon, Irthlingborough and Wollaston are shown in Figure 1 as having codes 92x. Hidden behind the scenes, code 956 was connected to the same selectors as 92. So Kettering customers calling Finedon could dial either the published 922 or if they knew it, the longer 9562 which would be pointless anyway. This teeing of codes allowed Finedon to have the STD code 093362 which translated to Kettering 9562. Some of the other charge groups on Figure 1 are similarly treated, 0801 translates to Kettering 94 and 0832 to Peterborough 92.
Northampton has a 'Northampton Ring' Code 0601 which translates to Northampton 8 and similarly 'Peterborough Ring' 0731 translating to Peterborough 8. The use of ring codes was intended to allow exchanges to expand in size and grow in number length. During analogue modernisation they were scrapped in favour of linked numbering schemes. Of the fourteen small exchanges surrounding Northampton, five had two digit codes beginning 8x and nine 82x. If 0604 had solely been used, the five exchanges would be limited to a 4 digit number and the nine to 3 digit numbers. Using a ring code 0601 allowed for an extra digit in the customers number, allowing a theoretical 10 times increase in customers. Oddity 0801/0832 is a case of one charge group with two STD codes, due to it being split across two parent GSCs of Kettering and Peterborough.
Outgoing STD Calls
When a customer in any of the three charge groups served by Kettering, dialled the initial 0 of a STD code, they would connect to register and translator equipment in the Kettering Group Switching Centre (GSC). This examined the next three digits (known as ABC digits) to determined the routing and charge rate for the call, which might be different depending on the originating charge group. The translator returned up to six routing digits back to the register which meanwhile stored the four to six digits dialled after the ABC digits. The register sends the routing digits followed by the stored digits into the trunk network, which is completely separate from the routes shown in Figure 1.
The trunk circuits from Kettering in the early seventies are listed in the table below.
London Faraday / Mercury
Birmingham Colmore Lodge
Kettering I/C STD
The code 20 was used should a customer choose to dial the STD code for a local number, the translation for 0536 was 20, dependent Charge Group codes 0801 and 0933 were routed similarly 2094 and 2095. Other codes starting with 2x gave access to nearby GSC. Codes 5 & 9 for the London or Birmingham director areas respectively. Code 7 took calls to the West Midlands towns, such as Dudley, Evesham, Hereford, Walsall, Wolverhampton and Worcester.
Leicester trunk exchange routed the majority of Kettering's calls to the rest of the country as well as Edinburgh, Glasgow and Liverpool. Cambridge Trunk exchange routed calls destined for the East of England. As Cambridge and Leicester were trunk exchanges, an additional digit was required to access their own customers, for example the translation for Leicester 0533 was 3•2 and Cambridge 0223 was 25•0.
Although Peterborough and Northampton were not trunk exchanges, like all GSCs with STD they had their own trunk selectors, these could be accessed from other GSCs such as Kettering by dialling a 1 into the exchange. At Kettering, the Northampton Charge Group 0604 translated to 21 and the customer's number. Coventry 0203 routed via Northampton with the translation 21•1•45 and the customer's number. Likewise Banbury 0295 translated to 21•1•37, Luton 0582 translated to 21•1•43 and Leamington Spa 21•1•02. We can deduce some of the Northampton trunk codes, Coventry was 45, Banbury was 37 and Leamington was 02.
Although I am focussing on the area I have documented in my notebook, similar modernisation programs were happening throughout the country, so this can be seen as a typical arrangement.
Figure 1 represents the routings prior to July 1971, before the first of many Unit Automatic Exchanges (UAX) parented on Kettering were modernised to Electronic Exchange type 2 (TXE2). Rothwell was the first to modernise this way, on 8th September 1971, its old 3 digit 'Rothwell' numbers being prefixed with 710 and becoming a Kettering number. This was the start of the Linked Numbering Scheme (LNS) policy which aimed to simplify local dialling procedures. In a LNS all exchanges simply dial the required number without the use of a code. The old code for Rothwell (76) was connected to a changed number announcement.
My apprenticeship notebook reveals, on 23rd August 1972, Desborough followed suit, prefixing its old 3 digit number with 760 and becoming Kettering numbers. The 76 part of this prefix was the old Rothwell code. Desborough's old code (74), was connected to changed number announcement until 6th December 1972, then made spare and connected to number unobtainable.
The next stage saw Great Oakley crossbar exchange open, as a relief for Corby which was growing on its southerly edge. This added the 74xxxx numbers to the linked numbering scheme and incorporated Geddington Cross UAX, by prefixing the old number with 742, freeing up the 79 code. After this Broughton UAX was converted to TXE2 by prefixing the old number with 790. The last TXE2 conversion in the charge group was Rockingham, its UAX's old numbers were prefixed with 770. The UAX serving East Carlton was closed by extending its customers lines to the new TXE2 exchange at Rockingham, its 3 digit numbers were prefixed with 771. Brigstock and Cranford remained outside the LNS until digital modernisation brought them into the scheme.
Kettering town had 4 and 5 digit numbers on its Strowger exchange, but due to the growing penetration of telephones during the seventies and a delay in constructing a new building, a relief mobile Strowger unit 'Wadcroft' was introduced. Later on, four mobile TXE2 units provide further relief with 6 digit numbers in the LNS with the format 51xxxx. When a much larger Electronic Exchange type 4 (TXE4) came into service at Kettering all these lines transferred over.
Designing the numbering schemes for a modernisation program was a highly skilled task, usually performed by the regional long term planning organisation. The TXE2 modernisation program hopping around the exchanges freeing up codes to be reused later was quite typical of the strategy. In Kettering town's case, the 4 digit numbers 2xxx to 4xxx were prefixed with '51', forethought set the relief mobile TXE2 numbers in the range 516xxx to 519xxx, so they were able to transfer to the TXE4 without a number change or clash with the four digit modernisation. The 5 digit numbers were prefixed with '5' becoming 581xxx to 585xxx.
Linked numbering schemes certainly made it easier for customers to dial within their local area. However there was quite a lot of opposition from villagers or remote towns losing their identity. As a compromise, although part of the LNS, some towns and villages retained their own name. For this reason, old dialling code booklets often show a number of towns appearing to have the same dialling code.
The red lines in Figure 1 show a few of the surrounding inter-GSC routes. Figure 2 removes the chaff of local exchanges and shows a wider area with dependent charge groups shown by arrows. In most GSC's no attempt was made to prevent through dialling, often because it was necessary for legitimate routings. It was possible to exploit this to make a long distance call at the local rate. However the further one travelled by this means, the fainter the conversation became, but 30-40 miles was possible.
The exploit was performed by joining local codes to one another. Using Kettering as the centre of the universe, official codes to Oakham and Northampton could be exploited, but not Market Harborough as through dialling was barred.
The longest route that I attempted was to a test number at Hunstanton (0485 3 xxxx) on the Norfolk coast, 81 miles by road. The test tone was too faint to hear but the repeated answering and clear signals confirmed the connection was made. The route from Kettering via Oakham and Stamford (a legitimate local call) then on via Peterborough, Wisbech and Kings Lynn to Hunstanton was 94 miles and should have been charged at 'C' rate.
Digital exchanges always sent the whole national number around the network, even to nearby exchanges. The data for the early exchanges allowed customers to dial the old analogue short codes. But a later change in national policy saw these short codes removed before most exchanges went into customer service.
The nineteen seventies term Phone Phreaking ( Freaking ) was used in the tabloid press for people exploiting the telephone system to make cheap or even free calls. This generally required insider knowledge and in some case illicit wiring inside a telephone exchange, a crime that would have lost the culprit their job.
One technique involved the use of a box to generate a 2280 Hz tone, which was held against the caller's microphone. This tone was used internally within the UK analogue trunk network. The freaker would dial an inaccessible code, then use the tone box to clear the distant end of the call, then use the box to redial the required number. The phreaker needed to have been supplied with the routing codes used within the trunk network, which had to come from an inside source. Examples of trunk routing codes are shown in the 'Outgoing STD paragraph above. Details in the press at the time, suggested these codes were readily available and circulating amongst university students.
The UK numbering plan developed for the introduction of STD in 1958, still forms the basis of our numbering in 2021, although many changes have occurred along the way. Back in the early nineties in the days before the Internet, the idea of a personal number that we could use anywhere on any landline or switchboard extension was muted, but never materialised. Perhaps in future years something might be developed, perhaps not a personal number but a handle, as we have on social media.