Hacking into VoIP systems is very simple once you know how it works. Nevertheless, hackers can be blocked by implementing countermeasures that were mentioned in the thesis. Building a VoIP system requires one to read the above three books. They give a lot of information on how VoIP systems can be easily hacked and they provide ways to mitigate them. Depending on the nature of the business one is in, the countermeasures may not always apply.
The following are interesting stories on how VoIP systems get hacked.