How can I use the web service with one or none arguments?
Do you have an 'index' operation?
Take a look at the services.services.api.php file in services module, docs folder.

In the node resource, you have several different 'operations' ( retrieve, create, update &amp; index, .. ).
Here's the snippet you should look at:

$node_resource = array(
    'node' =&gt; array(
      'operations' =&gt; array(
        ..
        'index' =&gt; array(
          'file' =&gt; array('type' =&gt; 'inc', 'module' =&gt; 'services', 'name' =&gt; 'resources/node_resource'),
          'callback' =&gt; '_node_resource_index',
          ..
        ),
      ),
    ),
  );
  return $node_resource;


GET request to endpoint + resource will trigger 'index'. Read more about it here https://drupal.org/node/783254