There are two possibilities:
1. You can add to your core a routable ip. When open your browser from the phone and go to http://<your_ip_address>/pluto-admin/check.wml.
2. If you don't want to add a routable ip to the core, you will need to use another linux machine which has one. There, you'll have to create a file "check.wml" which uses 'curl' to access the wap page from the core:
Header( "Content-type: text/vnd.wap.wml");
$defined_vars = get_defined_vars();
$user_agent = @$defined_vars['HTTP_USER_AGENT'];
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $serverAddress);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_USERAGENT, $user_agent);
There, you will be able to access the wap site from your phone's browser like this: http://<your_routable_ip_address>/<the_path_to_wml_page>/check.wml.