How do I access a function provided by a service resource module? Just getting started with Services so this is probably an easy question.

I'm attempting to allow users to access their private messages from a web app using this sandbox module.  At this point, after referring to the documentation on testing resources, I can get a JSON collection of the messages when I visit http://example.com/drupalgap/privatemsg, and I can get a specific thread if I visit http://example.com/drupalgap/privatemsg/threadID.

I understand the basic syntax is example.com + endpoint (drupalgap) + resource (privatemsg) + query.

However, the sandbox module includes a function for getting the number of unread messages, which I would like to do.  Here's the code:

 /**
 * Get the number of unread private messages of the logged-in user.
 *
 * @param int $uid
 *      User ID.
 *
 * @return int
 *   The unread count.
 */
function _privatemsg_services_unread_count($uid = '') {
  global $user;

  if (!user_is_logged_in()) {
    return services_error(t('This user is not logged in.'), 403);
  }

  /*
   * If a user id other than the current user's ID is passed, validate that the
   * authenticated user has the correct permissions to read another user's
   * messages.
   */
  if (is_numeric($uid) &amp;&amp; ($uid != $user-&gt;uid)) {
    if (user_access("read all private messages")) {
      $account = user_load($uid);
    }
    else {
      return services_error(
        t('This user does not have permissions to use this service.'),
        403
      );
    }
  }
  else {
    $account = $user;
  }

  // Return unread count.
  return privatemsg_unread_count($account);
}


My question is, how do I call this with services?  I've tried things like http://example.com/drupalgap/privatemsg/unread_count/ and http://example.com/drupalgap/privatemsg_unread_count but I get no results.
Functions are exposed via service resources, by a module implementing hook_services_resources().

You should take a look at whatever module your using to provide Services functionality for the private message module, specifically its implementation of hook_services_resources().

If you see an entry with a callback of _privatemsg_services_unread_count, then the function you want is available via a resource. If not, it currently isn't, and you can implement hook_services_resources() yourself to expose it.