When you're troubleshooting voice calls, particularly the signaling, it's often helpful to look at the detailed output of the device, be it a voice-enabled router, telephony server, gateway or whatever. This detailed output can be in the form of logs or debug output as is the case in Cisco's IOS. In any case, this output is likely to be somewhat cryptic. One of the reasons is because this output often uses a set of standardized codes and it is simply easier to put more information on the screen using a short number than it is to write an essay about why some event happened.
The good part about this is that the codes are often standardized, which means that your life may be a little easier in a multi-vendor environment, and if your vendor's documentation isn't fantastic, maybe someone else's offers a little more insight and will still be appropriate. The downside of course, is that you have to look the codes up to find out what's going on. This can be time consuming.
In any case, you should prepare yourself for this inevitable troubleshooting activity by looking at what kind of codes your device displays and finding the reference for them. Bookmark it or consider printing it out and tacking it to your wall.
Some links to the more common codes are:
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.