Ok then, with the proviso that this is all from memory, and I was trained as a telephone engineer when they were still extending the old clockwork (well, electro-mechanical, bit it still required a LOT of spring adjustments) exchanges, common control exchanges used either wire 'fingers' (Crossbar) or read switches Telephone eXchange Electronic (TXE) and the first all digital exchanges were still being developed.
Ok. Lesson 1
There were two fundamental kinds of local telephone exchange architecture in the Strowger automatic era, director, and non-director. The older of these two was the non-directory and is the basis of the UK numbering scheme. The director exchanges were a slightly later version of Strowger and were installed in the big cities. You’d recognise these as the 0XX XXX XXXX numbers in Manchester, Birmingham, Liverpool, Glasgow, Edinburgh and of course London. It was rumoured that Leeds and Bradford were going to be the next conurbation to receive a director system but the plans were overtaken by technology.
Lesson 2
Non-director numbering (as I was apprenticed to the post office telephone department in Bradford, this is the one I am more familiar with)
Before automatic exchanges (aka 'autos') numbering schemes were governed by the population size, and were 3 or 4 digits long. Exchanges were called by their town names. When auto came along the local numbers remained the same and each Telephone Area introduced local exchange dialling codes dependent upon traffic between specific exchanges with less common routes requiring longer local dealing codes and more common routes requiring shorter codes. Just about the only common feature in all these codes was that the parent Group Switching Centre was based in the local city and would be reached with a singe 9 (because that’s were the emergency operators were based and the first 9 of 999 took you there. It made things a LOT simpler for alsorts of technical and practical reasons. 0 of course, took you to the same switchboards, but didn’t light the read flashing light. Trunk calls needed an operator to assist as they needed to connect you via the truck network.
Lesson 3
STD. No, nothing to do with social diseases, and no, not Straight Throgh Dialing, but Subscriber Trunk Dialing. (No one owned a phone, but the Post Office and you subscribed to use the service. This was still the case when I started, and customers were generally referred to as 'subs').
Now this is where it gets both messy and simpler at the same time. Remember the brief mention of Director exchanges, well because of the densities of the areas they served a structure was very easy to impose. Each unit had a theoretical 9999 4 digit numbers it I don’t think they were all used) and each unit had a 3 digit identity. Quite early on they realised that by adding letters to the dial, these digits didn’t need to be random and they could be a represented as a three letter 'pointer' at the geographical location of the unit. This idea was carried over to the STD numbering schemes as they were introduced, hence Bradford was 0274 and Brighton was 0273. No where near each other geographically, but alphabetically very close in a list of the GSCs at the time. These were what we referred to as the 0ABC digits. Next came the local area Dialing code from the GSC to the local exchange, which could be 1, 2 or 3 (or even 4) digits and finally the actual subs line number of 3, 4 or 5 digits. Although early on it will only have been the cities where you’d get 5 digits.
Because for the way that the telephone network grew almost organically, and because of the way local dialing codes were allocated according to traffic levels, it tended to be that bigger exchanges with longer subs numbers had shorter local dialing codes so on the whole, STD numbers tended to even themselves out to mostly 10 digits. Not a hard and fast rule, but mostly.
Lesson 4
Of course, in an early implementaof Moore’s law* telephones got cheaper and people became more affluent so more telephones were installed and more telephone numbers were required. In the early days of expansion (where Director exchanges weren’t introduced to meet heavy demand) exchange 'mults' (their number ranges) were increased by the simple expedient of adding an additional digit to the front of a number. You might have experienced this in the bigger towns and cities as the numbers were increased from 4 to 5 or 5 to 6 digits. But then some standardisation came along and it was decided that subs would have a 6 digit exchange number and an STD code of 3 digits (the first 0 isn’t counted) and that local dialing codes would be phased out. The STD codes were already 3 and sometimes extended to include the local dialing codes. This was trimmed back to three and the subs number extended to include the local dialing code (where possible) Because of a combination of planning and serendipitous traffic requirements as described above, this was as far as the subs were concerned just a moving of a comma, and an education program to get them to dial 6 digits when they were calling Edna next door but one, instead of the 3 or 4 they were used to. Some changes though were more painful as changes had to be made where clashes would occur, especially where 5 digits were already in use. It also meant that some local codes between GSC areas that had been heavy traffic routes had to be withdrawn and the calls would need a full STD code to be dialled.
So that left us with 2 basic numbering standards, Director 0AB CDE XXXX where 0AB was the city CDE the unit, possibly geographically significant and XXXX the subs number AND Non-Director 0ABC DEXXXX where 0ABC were the city number, mostly geographically significant, DEXXXX the subs number, but DE being a specific town exchange. At this same time BT (who had just been created) decided to remove letters from the dial** /keypad because it was getting increasingly difficult to make the relevant numbers geo significant.
*might not be totally true
** This proved to be a mistake. A few opportunities were missed as a consequence of this, although the mobile letter to number allocation is different to the old dial layout, so it’s perhaps as well in the long run because it would have been hellishly confusing for everyone. We just need to get computer and telephone number pads the same way up now.
Lesson 5
Oftel/Ofcom and Massive expansion.
Not long after the creation of BT, Oftel was born and they took ownership of the UK telephone numbering schemes. The first big change was already in the pipeline and that was the splitting of London into two codes by making London the same as other Direcor areas and introducing additional digits at the front so 01 became 071 and 081 depending of your location and more importantly to BT allowed a doubling of the London numbers but kept the same logic across the board. Then a bit later, Oftel decided that competition would benefit if they (Oftel) could give different competitors different number ranges and the bast way to do this was to add another digit in front. Completely contrary to Oftel/Ofcom normal logic, BT was given the digit 1 as a prefix and other operators given other digits. This wasn’t a strategical thought out plan because London by this time was growing even faster than any reasonable expectations and the numbers were going to run out again. But at this time all other areas in the uk in the BT network also received the prefix of 1. (Remember, the leadings 0 isn’t counted )
So when London did threaten to run out of numbers again, a whole new scheme was decided on and 020 became the code, but a prefix was added to the three digit unit code, but by this time I was a long ways away from my humble roots as a telephone engineer and I thing I was worrying about the millennium bug.
So that’s a potted history of how we got to where we are today, but of course with the advent of mobile phones we’ve all got used to numbers not really signifying location, or indeed any specific operator. For the time being however, number portability on the fixed line network is being resisted, though whether that’s the operators or Ofcom, I really couldn’t say. Certainly, it’s been technically possible to point any number at any network point since the completion of the digitalisation of the network as the number is only name and the actual address it is translated to is flexible. Though whether or not it’s worth the complexity of introducing such a scheme given the rat that PSTN traffic is falling is perhaps debatable. Universal VoIP i recon will be here within the next decade.
I realise that my writing style is a bit of a core dump, so if you’ve got any questions I’ll try and answer them
