Get the Whitepaper called Cisco VoIP Phone – A Hackers Perspective.
You can download it from the following link: https://packetstormsecurity.com/files/download/92154/CVP-HackersPerspective.pdf
Source: https://packetstormsecurity.com/files/92154/Cisco-VoIP-Phones-A-Hackers-Perspective.html