Basic troubleshooting tools for VoIP networks include packet sniffers like Ethereal, log analysis, a softphone like X-Lite, and a VoIP server like Asterisk that allows detailed debugging output in its logfiles
Ethereal has built-in parsers that allow you to easily view SIP methods and responses, as well as SDP attributes and capabilities negotiation signals
Ethereal's packet capture filters are configured using a string syntax. This syntax is described in Ethereal's documentation and in O'Reilly's Managing Security with Snort and IDS Tools
Packet capture often reveals what is going wrong, but you may have to investigate diagnostic output on both ends of the signaling conversation in order to find out why
X-Lite's diagnostic log records every SIP packet it sends or receives
Asterisk uses an internal thread identifier that doesn't necessarily correspond to the SIP call ID, but can aid you in tracing an individual call through Asterisk's logs
Troubleshooting quality-of-service issues is aided more by process of elimination than it is by packet analysis
Network General Sniffer and WildPackets EtherPeek VX are packet capture tools that offer inspection of RTP traffic if needed
IPerf can let you simulate traffic loads on a call path in order to determine its true VoIP performance capability
