How does your network carry voice?
This measures this browser and this device's path to this endpoint, right now - the device, the browser's media stack, your access network, the ISP, transit, and the endpoint itself. It is not an ISP verdict and not a speed test.
Sends and receives a few megabytes of synthetic audio and probe messages. No microphone access. A 30-second window cannot observe rare events; treat a clean result as "clean for 30 seconds", not "clean".