As discussed on the call subject page, the FastCall widget can be customized in a few areas. The User Interface settings control the notes box; gives an option to copy the custom field for FastCall notes (short text) to the native Salesforce field for comments (long text.) FastCall short text notes can be used in a related list. Last, you have the option to remove or show the call subject.
The next section for FastCall widget customization gives call termination options. Here you control whether your users will see the call result picklists. You also control how calls are ended.