When people talk about monitoring Service Level Agreements or SLAs in the world of VOIP, they are almost always referring to measuring packet delay and jitter. More specifically, they're talking about the delay and jitter in the packets carrying the RTP protocol, which is in turn carrying the actual voice signal. Obviously, this is important because the quality of the voice sound is what users notice most. However, you should also set SLAs and monitor the signaling and call-management protocols. This signaling is what sets up and tears down your voice calls and handles such functions as call transfer, hold, 3-way calling, etc. It also handles the ring and busy signals you receive.
In a large environment, as the load on a gateway or gatekeeper or SIP proxy increases, you may see longer setup times or more variance in the time it takes to set up calls. So naturally, it's a good idea to assign reasonable time limits to these functions so that you can be prepared to upgrade your equipment or deal with any problems when the actual values approach your limits. And this is imperative if you're relying on another party, such as a vendor or service provider. In this case, you'd want to set SLAs for these metrics and be able to watch them closely.
An example of a software package that can set thresholds for H.323 and SIP operations and notify you when they're exceeded is AppDancer/FA, which I think stands for "Flow Analyzer". It does a pretty good job of graphing the packet-level signaling communication so that it's easy to understand. This also helps you troubleshoot when the signaling completely breaks down.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.