In article <20080125140553.GA32299@nic.fr>, Stephane Bortzmeyer <bortzmeyer@nic.fr> writes
in the UK it [phone number portability] 's done with something similar to DNS. The telephone system looks up the first N digits of the number to determine the operator it was first issued to. And places a query to them. That either causes the call to be accepted and routed, or they get an answer back saying "sorry, that number has been ported to operator FOO-TEL, go ask them instead".
What happens when a phone number is ported twice, from BAR-TEL to FOO-TEL and then to WAZ-TEL? Does the call follows the list? What if there is a loop?
In the UK, for landlines there are generally only two operators available: BT and Virgin (the now sole brand for cable phones). So WAZ doesn't exist, all you can do is go back to BAR. For mobiles, I've never heard of a restriction so it's probably the case that the donor network stays the same, but the recipient records are updated to point to WAZ instead of FOO.
The solution you describe does not look like the DNS to me. A solution more DNS-like would be to have a root (which is not an operator) somewhere and every call triggers a call to the root which then replies, "send to WAS-TEL".
That's the scheme which was proposed in 2002, and which I'm a bit surprised isn't yet deployed (watch the space called ukporting.com [1], apparently). However, the current mobile scheme isn't very far off that. [1] Why not ukporting.org.uk ?? -- Roland Perry