The source computer hardware might be a limiting factor in running call simulations, as well as the destination device. Where possible make sure the systems are not contending for resources (CPU/Memory/Network). Virtual Systems are prone to be starved for resources. Physical Laptops and Desktops are the best suited for testing.
It is recommended that you shut down other applications to prevent contention caused by other software. (Outlook for example, will check for messages every so often and will cause a hiccup that could show up as higher latency as well as jitter). Consider Antivirus Software to make sure it doesn’t run a scan during a test and make sure there are no backups going on. You will want to have 100% of the Server’s attention to Voice Simulation.