If your call keeps getting disconnected with the dialer and there are no server or internet issues.
One reason is another application has higher priority of using your microphone. For some reason browsers, specially google chrome has lower priority when gaining access to your microphone. You can try the following to resolve this issue.
1. Disabling Exclusive mode on your Microphone devices. You can follow these steps to do that.
2. If that still did not work. Try uninstalling and reinstalling google chrome browser or use Mozilla Firefox browser.