There are some well known SIP extension enumeration vulnerabilities in different VoIP servers, specially in Asterisk. This brute-force vector is based on the study of the authentication responses of the target server. Sometimes its replies are different in the case that the client uses a valid extension, so it's easy to discover them. This vector is normally classified as a low security risk. Moreover we're moving towards a federated SIP environment , in which the extension is the public email address of the user. But it's still important in some cases: To guide next steps during a penetration test. In example, you can use the discovered extension to reduce the number of attempts in the phase of SIP extensión brute-force. Some RCE (Remote Code Execution) exploits need a valid extension to work. After a little research, these are the known vulns: CVE-2009-3727 : It's quite old and it's practically not present in real environments. It's still not imple...
... mind overflow ...