Analyzing VoIP with Wireshark

Nice tutorial to Analyzing VOIP with Wireshark, if you are familiar with tcpdump it is childs's play. Just get .cap dump using tcpdump

tcpdump -w dump.cap -p -n -s 0 "udp”

This will capture all udp data, you can filter the data according to you requirement. Which can be filtering by port etc..

For more read here
Panoramisk » Analyzing VoIP with Wireshark