How to make call out to secured web service

Am receiving the message "System.Net.WebException: The HTTP request was forbidden with client authentication scheme 'Anonymous'." when trying to call the web service at "https://graphical.weather.gov/xml/SOAP_server/ndfdXMLserver.php" using BizTalk 2013.

I've imported the wsdl directly from the site, creating the schemas and the bindings to make the call. The Send port is set up as WCF-BasicHttp, Security Mode: Transport, Transport Client Credential Type: None. I'm able to call the service using SOAP-UI from the BizTalk server, providing no form of authentication.

Have read numerous posts and documentation, but nothing I've done to this point has helped. I'm sure I'm missing something; just not sure what that something is!