In message , at 02:04:47 on
Mon, 20 Jul 2009, Charles Ellson remarked:

ISTR the exchange "owning" the number now rejects the call and
instructs the originating exchange where to send it (all done in
milliseconds) BICBW.

That's what they do for number portability. Perhaps it's also used for
out-of-area numbers, but I'm not aware of it.

A trawl of the OFCOM website suggests they only recognise "number
portability" in terms of mobile and 070x numbers.

There's an EU Directive that says all numbers must be portable.
Landlines are at the moment.

AFAICT their explanation seems much the same as how the System X
version was explained to me for "permanent diversion" which took over
on lines previously hard-wired to a remote location.

Currently number portability is implemented by the "old" exchange having
a list of numbers which have been ported, and forwarding them to the
relevant new exchange. This has many disadvantages and will be replaced
by a new "Direct Routing" system which interrogates a central database
to discover which exchange (and which telco) the call should be
delivered to.

The older version on some exchanges required use of a directory
number at the exchange actually serving the subscriber to which calls
were silently diverted by the exchange which "owned" the number; IIRC
that became unneccesary once everything was replaced by System X or newer.

Calls are still diverted. Maybe System X means you don't have to use up
a "mapping" number at the destination exchange any more.

Call diversion tends to be charged by use, whereas an out of area number
would be a flat rate.

It would not be the first time that the same service was sold at
different rates with different names.

Call diversion, as an explicit service, costs a lot of resource (eg
CPU). I'm speculating that the telcos can deliver an "unlimited" number
of diverted calls cheaper than running a leased line (and hence
implement it that way, today). But the customer probably prefers a flat
rate, rather than paying per call.
--
Roland Perry