Here is a link to a good video on VoIP and SIP security.
Patrick McNeil covers how to hack SIP servers, at service providers and enterprises, to make money. The presentation is useful because it covers why someone might bother to hack these servers, namely:
- Toll fraud or International Revenue Sharing Fraud (IRSF) - basically using someone elses PBX to generate calls to premium numbers set up by the hacker or where the hacker has an agreement with the owner to generate traffic.
- Telephony Denial of Service (TDoS) - using someones PBX to target a business or individual. The attacker makes money through extortion.
- Robocalling/SPAM - using someone's PBX to make many SPAM or vishing calls to individuals (this was briefly mentioned).
At the end of the presentation, there is a brief TDoS demo.
Note that another way to make money is call pumping, where the calls are to 1-800 numbers and the attacker gets a share of the revenue, but this arrangement is more difficult to set up.