The video/voice call is of poor quality sometimes.
If you are under WIFI connection, tap your profile on the upper left of your screen, go to Settings > Calls > Wi-Fi traffic mode to adjust the call quality. If you are using cellular data, choose 2G/3G/4G traffic mode to adjust the call quality.
Please make sure that you network connection is stable while making a call. Because a bad connection can cause dropped calls or frozen video, and poor sound.
If you have strong network connection at the time but the call quality is still poor, please go to "Any other questions?" for further help.