He’s mostly right with that, although he missed a bit about the technological reason why the big conurbations have three numbers and ho that relates to the 7 digit local numbers.
With the advent of STD (he miss out a whole lot of stuffs between operator control and STD, but that’s way beyond the scope of this reply*) National telephone numbers were standardised on 9 digits (ignoring the initial 0). Director technology, as implemented by Post Office Telephones, used 10000 ‘mult’ units giving 4 digit numbers, and 3 digit unit codes. This left 2 digits for the area/city codes. THAT London was of course a special case with an 8 digit number. So these areas had 7 digit local codes and and a 2 digit area/city code (9 digits in total)
Other cities/areas generally had 6 digit numbers and 3 digit area/city codes mapped to some geographical three letter code that made some sort of sense (9 digits in total)
Remember, this was the STD model plan. Those of you who are old enough will remember 3,4 and 5 digit local numbers with a perfusion of local dialing codes of various length. These were near.y always technology lead and based on the demand for a) the number of phones on an exchange, and b) the number of calls on routes between exchanges. All this had to be morphed into the 6 digit local number model which also did away with the need for a lot of the local dialing codes.
The digit 9 was nearly always the number used for the Main Switching Centre your GSC was parented on because this would most likely be the location of the emergency switchboard and it made 999 call routing a LOT easier. Director areas didn’t have GSC/ MSC architecture as they were register/translator controlled.
The Trunk Network (that’s the national intercity network) was also register/translator controlled, which allowed the STD model to exist. And then there was something called the Transit Network which overlaid the Trunk Network to allow faster switching times for busy routes. Again, way beyond the scope of this post.
Over time Ofcom have followed the models set up by the Post Office in removing ALL local dialing codes and educating the public to use national numbers by default. It’s also interesting to note that most of the number expansions that Tom Scott mentions in his video were foreseen by and planned for by the Post Office Telephone planners way back when and that’s why the original STD model easily accommodates those changes.
Please remember I spent five years learning this stuff on day release and training courses and a further ten immersed in the environment while most of the changes were being made. And it was 30 years ago since I moved out of telephones and more towards IT. It’s difficult to try and condense this stuff into short posts, let alone remember all the fine detail
