"John Shelley" wrote in message
news
Martin Underwood wrote:
snip
By the way, how much of a London number can you omit? You can omit
the 020 if you're calling from a London number but can you also omit
the district code (the next four digits) if you're calling another
number in the same district?

You have to dial the 8 digits. Think for a moment, how does the equipment
know that the 4 digits entered is a local number and not somebody having a
pause between "dialling" the exchange and the subs number.

Yeah, silly question, on reflection! I presume the equipment has to accept a
fixed number of digits (previously seven, now eight) and identify the first
four (previously three) as the district and the remaining four as the
subscriber number. If the stream of digits begins with a 0, an alternative
algorithm identifies from the digits that follow how many are the exchange
(eg "20" signifies London, "1344" signifies Bracknell). I can see that if
you only dial the final four digits, they could be confused with 0
signifying "what follows is an exchange" or 1 signifying special numbers
like emergency (112), directory enquiries (118xxx) etc.

By the way, how did changing from 0171 xxx yyyy or 0181 xxx yyyy to
020 7xxx yyyy or 020 8xxx yyyy help alleviate the shortage of
available numbers in London? It didn't increase the number of
available phone numbers - all it did was to change the mapping
slightly. OK, so there's scope for additional district codes
beginning with digits other than 7 or 8, but it's not districts that
are in short supply, it's subscriber numbers (the xxxx in the above
example).

In precisely the way you say. Instead of 2 x 10,000,000 numbers there are
now100,000,000. As to the point that it's not exchange codes that are in
short supply. but subscriber numers, all you do is add another exchange
number to an area, creates another 10,000 subscriber numbers. Many of the
4
digit exchange codes are actually located in the same building. Here in
Harrow the exchange building housed both the 8427 and 8863 exchanges and
probably others as well. With the arrival of electronic exchanges the
physical space needed for an exchange was vastly reduced so adding extra
switching capacity within a building that was built to house a mechanical
exchange isn't a problem. The extra exchange numbers are also needed for
the non BT operators.

Ah, so new suscribers in an area potentially get a brand new district number
that's unrelated to that of all the other subscribers in that area? Yes, I
suppose that's one way of solving the problem. Do all subscribers in one
area get one new code and all those in another area get different code: can
you still say "xxxx [a new code] is Harrow, alongside yyyy [the existing
code]" or is the code-to-location mapping lost?