HTTP status 407: Proxy authentication required Error when calling web service

Question

I have spent two hours on this error "HTTP status 407: Proxy authentication required Error " when calling a web service, my code is like following

            WebProxy oWebProxy = new System.Net.WebProxy(ProxyServer, ProxyPort);
            oWebProxy.Credentials = new NetworkCredential(ProxyUser,ProxyPassword,ProxyDomain);
            oserv.Proxy = oWebProxy;
            oserv.Credentials = new NetworkCredential(theusername, thepassword);

I have verified that the proxy address, user id password are all correct, and I could access the web service thru the IE in same pc, but when I run the code with VS, the error keeps popup. I have tried the UserDefaultCredentials=true as well, but no luck.

Any idea?