When we moved our phone back to BT from NTL (with a massive cost saving in calls to boot), the engineer that came to check the BT line said they
had to ensure the line was working before NTL released the line, and then
had to ensure that the phone number had been correctly transfered over - bascialy before he was allowed to go home... Something to do with emergency use....
What a shame NTL don't seem to follow the same rules.