Troubleshooting is a big part of working with FreeSWITCH and VoIP. There are so many moving parts, and so many of them are outside your control, that you will soon become fluent in debugging failed calls.
Usually you have a patchwork LAN where your users hook different phone models, then your FreeSWITCH server, then one or more ITSPs and/or DID providers. Interspersed, you have firewalls, routers, ADSLs, T1/E1s... And you often have direct control only of your FreeSWITCH server! Ouch!
This chapter will give you first the big picture, how it works and why it breaks, and then will introduce you to the latest and best tools for troubleshooting VoIP.
We're very lucky that FreeSWITCH is so reliable, predictable and well-documented. But the ecosystem of VoIP extends far into uncharted territory.
For a comprehensive view of the problem, I would recommend VoIP Deployment for Dummies, Wiley— it's not for dummies at all, and (even though it was published in 2009...