Using Wordpress phpUnit and wp_mock in vscode

I am new to php and experimenting with unit_testing and wp_mock. I have an issue which I am unable to resolve. Please could somebody point me in the right direction.

I have a basic class Plugin, within Rcc_Plugin I am trying to call the method in Test_Rcc_Plugin extends TestCase, as below.

 $rcc_plugin = $this->get_subject();

This method returns a singleton which contains the method register()

When I call $rcc_plugin->register() in the test; I get undefined method in vscode intelephense.

I've tried to find a way to solve this by checking namespaces etc. The rcc_plugin class is in another file, init.php.

class RCC_Plugin {
    public function register()
    {
        // declare the shortcode
        add_shortcode('rcc-django-data', array($this, 'load_events'));
    }
}