I've done a few seminars on troubleshooting VoIP at VoiceCon (Fall 07 and Spring 08). Somewhere along the way, someone mentioned a relatively new web site to me:
voiptroubleshooter.com
I finally took time today to look at it. The content is potentially
useful, depending on your level of expertise. If you're a long-time
VoIP troubleshooter, then you'll already know much of what's there (or
should). If you're getting started, it has some useful background knowledge and procedures. The main menu has items about VoIP basics, online diagnosis, tools and resources, and links for additional reading. It is not a lot of information, which is sometimes helpful when starting out in a new technology.
One of the things that I didn't see there were procedures for diagnosing problems specific to voice gateways or to DSP (digital signal processor) pools used for transcoding. In the Network Symptoms page, they have a nice matrix of the types of problems that generate unacceptable jitter, packet loss, or out of order packets (interesting, nothing there about unacceptable delay). In the matrix, I would have put Duplex Mismatch under Intermittently, not Continuously, because it is most often a factor when the link with the duplex mismatch is running at high enough loads that the duplex mismatch causes packets to get damaged. But still, it is a good start - more than I've seen in other places.
Under the Online Diagnosis link, there's a neat calculator in which you select the codec, frame size, and packet loss rate and it shows you the MOS score, R Factor, and required bandwidth.
This site may be a reasonable resource if you're starting to work with VoIP.
-Terry