In article , Richard J.
writes
Are you saying that all numbers on the same exchange will start
with the same 4 first numbers?
I guess not, but what do you mean by '4-digit exchange code'?
To take the number in the title of this thread as an example, the
enquiries number for London Transport used to be ABBey 1234, one of up
to 9999 numbers on the ABBey exchange, which became the 222 exchange
when all-figure numbers were introduced. That exchange is now the 7222
exchange, and all numbers on that exchange have the format 020 7222 xxxx.

Um, I'm afraid not.

The connection between exchange and number was broken in London much
earlier than elsewhere. The 7222 bit is technically known as a "director
group" after the technology used at the time (though not any more).

Each first-tier exchange (known as a "concentrator") is allocated one or
more blocks of numbers. These blocks vary in size; most commonly they
are 10,000 or 1,000 numbers, though I know of cases where they are as
small as 100 numbers. Most concentrators handle several separate blocks.
It is normal for all the numbers handled by a concentrator to be in a
single dialling code, but I won't claim that's universal.

020 7222 is managed by Westminster Unit G (interestingly, the internal
BT code for this contains "ABB" rather than "WES"). This manages the
following number blocks:

02032681
02072020
0207222
0207227
02076541
02076542
02076543
02076545
020765476
02076549
02077830
02077831
02077832
02077835
02077836
02077837
02077838
02079601
02079606
02079607

Concentrators are controlled by the second-tier, known as "DLEs"
(Digital Local Exchanges); there are an average of 10 concentrators per
DLE. Westminster G is controlled by Southbank Unit E.

It is common for a DLE to span several dialling codes *and at the same
time* for a dialling code to be split among several DLEs. For example,
the five or six DLEs in Cambridge handle all of 01223, most of 01954,
and good chunks of 01353, 01440, 01736, and others.

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