Get the Whitepaper called Sniffing VoIP Calls Using A Raspberry Pi.
You can download it from the following link: https://packetstormsecurity.com/files/download/157963/sniffing-voip.pdf
Source: https://packetstormsecurity.com/files/157963/Sniffing-VoIP-Calls-Using-A-Raspberry-Pi.html