When calling external web services from a Volt app in an environment that uses a proxy server to access external web sites, it is possible to call external web services via the proxy server by setting the proxy host and port in jvm/lib/net.properties of the Domino JVM.
http.nonProxyHosts=localhost|127.0.0.1|10.*. *. *. *|192.168.*. *
If that proxy server requires authentication, there is no parameter setting in the Domino JVM for that, so it is not possible to call external web services.
NotesHTTPRequest uses Libcurl to support proxy servers that require authentication by setting an environment variable.
If the Java code that sets the user and password in Java's Authenticator class and accesses the specified URL is run in the Domino JVM, the proxy server will authenticate and the specified URL can be accessed correctly.
If calling an external web service via a proxy server that requires authentication in this way, Volt should support authentication, as does Libcurl.