Get the Whitepaper called Enumerating and Breaking VoIP. It discusses various enumeration techniques followed by a demonstration of a few VoIP attacks.
You can download it from the following link: https://packetstormsecurity.com/files/download/107178/enumerating_breaking_voip.pdf
Source: https://packetstormsecurity.com/files/107178/Enumerating-And-Breaking-VoIP.html