There are three main calling methods in Voxofon application: Internet/WiFi calling method, Local call method and Callbacks.
To make a call with a desired call method you would need to set the call method to corresponding one.
To change call method navigate to "Outgoing" button at the right bottom action bar, tap on it, a pop up with "Connect calls using" title will appear with call methods. Tap on a call method to choose it and it will be selected.
Please consider that you should see only two calling methods, that could be "Internet/WiFi" calling method and either "Callback" or "Local call" method.
"Callback" method appears if "Local cal" method is not available for you.
If local call method is available for you, but you would like to use Callback instead, navigate to application "Account" section, Open "Voice calls" section and enable"Allow Callback" option.