Android logcat output can be enabled in the following way:
Enable USB debugging on your phone first as described here (only the first two sections are relevant: "Enable Developer options" and "Enable USB debugging on your device").
Then install Android Studio on your PC/laptop, connect your phone via USB, reproduce the problem and send the logcat output.
(Android Studio should detect the connected device and you just need to click on the logcat icon to see the logs. You can filter the logs to "MizuDroid" or to your app package name).
Alternatively, if you don't wish to install the full Android Studio, you can install only the SDK Platform-Tools and use the adb logcat command.
Make sure that the logs will include both the app startup and the call with the problem (including the call disconnect).
If your logcat crops the logs after some time, then you should increase it.
From Android Studio you can set the "Logcat cycle buffer size" to 20000 KB. You will find this at Android Studio -> File menu -> Settings -> Tools -> Logcat (or just search for "logcat" in the settings)
Send the logcat content to info@mizu-voip.com (attach as file, don't copy-paste into the email body), together with an accurate problem description.