Telephone Charging Rates (from BT) - a description of how to determine what is and isn't a local call, and an area-by-area listing of where is local to any given number.
A description of the BT network - explaining the difference between a DLE and a DMSU, and listing the main network switches.
Some notes on France Télécom charging rates and the French renumbering of 1996
The information source is the Telecom Digest archives, which also has an external links page. Another good summary page is the World Telephone Numbering Guide.
The UK telephone system is regulated by Oftel. There's a lot of useful information at Warwick University. BT still control much of the technology via SINS (Supplier Information Notices).
SEG Communications have sponsored a nice site about Strowger Telecoms.
The North American Numbering Plan (that is, the area codes in Canada, the USA, and various Caribbean nations) is maintained by Telcordia, who were Bellcore.
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