On the Internet, the round-trip latency of a phone call can be relatively stable from one moment to the next but vary heavily from one day to the next . You'll use troubleshooting tools to find the sources of latency that can make VoIP call quality fluctuate.
On the Internet, moment-to-moment changes in latency are called jitter. Jitter is a huge problem. You'll use troubleshooting tools in order to identify the root causes of jitter.
Signaling and interoperability problems
Perhaps the most difficult to isolate, interoperability problems require a working knowledge of the signaling protocols you use on your VoIP network. Avoid interop problems by sticking with a single standard, or even a single vendor, if you can. But when interoperability is required, you'll use troubleshooting tools to isolate problems.