PathSolution’s TotalView validates that the switches and routers are not dropping packets and also helps show how things are connected. If everything is healthy there, then you can quickly rule out a whole set of issues.
In general, you want to do troubleshooting with only 1 call. You usually want to simulate more than 1 call only when you are doing network validation testing to see if you can handle the load of X number of calls simultaneously.
Note: If you run the call simulator from a computer, make sure it is not a virtual machine. Virtual machines do time-slicing between the processors that will mess up the timing for call simulations and show high latencies and jitter when the network is fine. The destination location should also be a physical computer and not a virtual machine. It can be a computer, router, or switch, or even phone. Generally, if you want the best (most accurate) results, you should test from a desktop computer to another desktop computer. You will want to make sure that no other processes are running on the computers that could interfere (Outlook for example will check for messages every so often and will cause a hiccough that could show up as higher latency as well as jitter).