British Telecom (BT) operates an automated fault detection and reporting system that allows anyone to test any line. If the line is found to be faulty the caller is given an option to divert all incoming calls for that line to another number, including mobile phones. No authentication is required and the owner of the line will be oblivious to the fact that her calls are being hijacked.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/41060/bt-hijack.txt
Source: https://packetstormsecurity.com/files/41060/bt-hijack.txt.html