Blitzz video call doesn’t require a high CPU. Processing video in real-time means encoding and decoding it, which tends to be resource-consuming so you may hear the fans spin faster to keep the CPU cool - this is very normal.
If you see this appear randomly, try to restart your computer before a call. Other software might be using computer power or interfere with your video or microphone. Restarting your computer will assure your computer is ready for video.
If you continue seeing this, try to analyze your task manager (Processes and Performance tab) as in the image below to look into this further. Hit CTRL+SHIFT+ESC (on windows).
How to access the task manager.