Richard J. wrote:
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.

There is no such thing as an 'exchange code'. Telephone numbers are
virtualised and are not tied to particular lines or exchanges although,
for historical reasons and ease of management, numbers will correlate
strongly with particular exchanges.

ESB