[Junk Box Nick]

This is fascinating stuff.

I long ago figured that there was some sort of alphabetical sequence to the 0789/01789 numbers and can recall pre-internet when only having the telephone number of a firm that wasn’t on a ‘local to me’ code and wanting to know where it was located going to a telephone directory to look up the area code. As the areas were listed first I would look towards the end of the list if the code started with a high number.