This error particularly occurs if you are behind a firewall or proxy.
When you use a winforms app, it can autodetect the proxy using IE
settings for current user, and it can connect out through the proxy.
However, when in asp.net, it cannot because the asp.net user identitiy
does not have the correct proxy settings.
To resolve this issue, you need to explicitly specify the proxy settings
for your application. You can do it at various levels viz., at the
Machine.Config file which will apply for all the applications running on
the system, at the Web.Config file such that it applies for a single
application, and at the Page Level programattically in the code behind /
For Machine.Config use the same settings as above.
To do it programmatically,
WebServiceClass MyWebServiceClass = new WebServiceClass();
WebProxy proxyObject = new WebProxy("http://address:port", true);
MyWebServiceClass.Proxy = proxyObject;
where WebServiceClass is the name of the Web Service Class which you are consuming.
That should solve the issue with consuming the Web Service across Proxy Settings.
Hope this will help you