Thanks for the reply. I can successfully connect to it using netcat via command line on my machine. But trying 'fsockopen' on my web application also results in "No route to host". I've checked the firewall modsec log but there aren't any corresponding entries.
Might have to pass onto our system...
I've been unsuccessful in trying to connect to the API (via PHP) using the methods commonly found online.
e.g.
$client = new \PleskX\Api\Client($host);
$client->setCredentials($username, $password);
$customers = $client->customer()->getAll();
and
$client = new PleskApiClient($host)...