Viproy is a tool for testing SIP servers security, the Session Initiation Protocol is widely used for voice and video calls over IP, the software comes with different modules performing specific tasks, all of the modules support debugging and verbose mode, this is a Linux only command line tool, instructions are included and it should not be difficult for a Linux beginner to understand them.
Software modules consist of options, register, invite, enumerator, brute force, trust analyzer and SIP proxy, you can set target networks and port numbers. Before carrying out any attack you should fingerprint and enumerate SIP services first, after that you should register with the server and start intercepting, making calls or create havoc at will.
Viproy VoIP penetration tests include targeting a local client address and port, discovering SIP services with valid credentials, setting username and password in Asterisk PBX, issuing direct invites and spoofing without credentials, enumerating all users, launching a denial of service to all valid users so that nobody can accept calls and brute forcing a target account or numeric range using a dictionary list to test users password strength.
Viproy homepage lists a vulnerable VoIP server where you can evaluate your hacking skills without harming anybody, in a real life scenario after successful hacking a VoIP server you can listen in or record inbound and outbound calls as well as setting up usernames and passwords, the damage that can be done will depend no how many vulnerabilities exist, not all of the modules will be necessary successful penetrating the server.
Another tool you might want to add to your VoIP hacking arsenal is SIPVicious suite you can use it to audit VoIP systems scanning SIP devices IP range and cracking SIP PBX. VPN services protect VoIP calls in transit but the first and last point remain vulnerable, it is possible to listen in to a VoIP encrypted call by hacking into a server before encryption takes place or when the call is decrypted at the end of the line.