On Tue, 3 Apr 2012 16:20:44 -0700 (PDT), Owain
wrote:

On Apr 3, 8:39*pm, "Adam H. Kerman" wrote:
Suppose we'd used a system like this for telephone numbers. Then exchanges
serving areas with lower populations could have issued shorter line numbers.
If the overall number length was to be the same a la ISBN, then the small
exchange codes themselves could have been longer.

which is, to some extent, how the British geographic numbering system
works. Number are 0+10* eg

(020) xxxx xxxx London
(029) xxxx xxxx Cardiff
(0113) xxx xxxx Leeds
(0116) xxx xxxx Leicester
(0131) xxx xxxx Edinburgh
(0151) xxx xxxx Liverpool
(01382) xxxxxx Dundee
(01386) xxxxxx Evesham
(01865) xxxxxx Oxford
(01792) xxxxxx Swansea

(01204) xxxxx Bolton *
(015396) xxxxx Sedbergh
(016977) xxxx Brampton


While number allocations can often be resolved to smaller blocks
matching older small exchange areas ITYF there are no longer any 4- or
5-digit directory numbers, such having been absorbed within the
general 6-digit numbering schemes usually identified by the main
exchange in a group (or the name of main city/town). It is mostly a
matter of convenience that the old allocations are usually adhered to
but if the numbers run out then there is generally no bar to using
number groups "robbed" from somewhere else in the same exchange group
as used to happen in Strowger/clockwork days either by extending
individual lines over junctions between exchanges or by routing calls
at an intermediate level over junctions from one exchange to another,
e.g. :-

N1-----N2------final selector 21xx "donor" exchange
|
junction
|
| **Exchange with number allocation used up**
|-----final selector 22xx "donated" range

N1-----N2------final selector 67xx allocated range

("donor"/"donated" used for convenience)
Where this arrangement involved a degree of permanence the donated
numbers would sometimes be allocated an exchange name either
non-specific or distinctive (often if they all served the same
establishment or area) which would be identified in records as a
"hypothetical" exchange thus providing a warning if e.g. somebody
complained that "when I [incorrectly] call ABCtown 2234 I go through
to DEFtown 2234".

The use of translation in director areas allowed other "fiddles" such
as having what was in reality a 5-digit exchange (i.e. N1st, N2nd,
N3rd, final selector) but with the first numerical selector receiving
the final routing digit. A similar effect was achieved at some
exchanges in central London by inserting an N3rd selector before
11-and-over final selectors (which operated off the first digit
received and discarded/ignored the final digit) thus increasing the
available directory numbers in the affected ranges by tenfold.

Confusion is avoided by always dialling the 0 for long distance calls,
and for calls being charged correctly regardless of whether the code
is dialled or not, for own-exchaneg calls.

Owain



* there are a few exceptions where a 01xxx code is followed by a 5-
digit subscriber number