Using a headset with a microphone or earphones will improve sound quality and reduce echoing. Make sure you do not have your volume levels set too high as this can cause echoing on the recorded video. Also make sure if you are using a phone or tablet that you have it in portrait mode. The use of USB style echo-cancelling speakers where headsets or earphones are not available will also help reduce echoing if you using a computer or laptop. Ensure your internet connection meets the following bandwidth recommendations - video 300kbps per stream, audio 50kbps per stream.
You can perform a "pre-call" test which will check your hardware, and internet connectivity. It may not guarantee your hardware will work but will provide an indication.
Simply click "run test" from this link.
The faster and more stable your broadband connection is, the better the video quality will be. If your bandwidth drops below a certain threshold, especially if it’s being throttled, or if you have very little bandwidth, behaviour can be unpredictable. The interview may be choppy and sound may have artifacts, and the connection could be dropped.