Some cases of video call problems are caused by the wrong phone application. To troubleshoot the application, you can perform three operations: force shutdown, clear cache, and clear data. Make sure to force close the Phone app before clearing its cache or data
To fix this follow these steps:
Ensure that the camera and microphone of the device are not physically obstructed.
Check whether the camera and microphone are in the correct direction.
Check your phone or Wi-Fi connection.
Please restart the app and try again.
Restart the device and try again.
Check the battery level of the device.