Asterisk Project Security Advisory – The Asterisk maintainers have made it so that a scan for valid SIP usernames always returns with the same response.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/76307/AST-2009-003.txt
Source: https://packetstormsecurity.com/files/76307/Asterisk-Advisory-Information-Leak.html