Proxy Authentication Required Issue

Hi all,
My web application consumes a web service with the following code.
System.Net.WebProxy objprox = new System.Net.WebProxy("http://proxyservername:portnumber");
objprox.Credentials  = new System.Net.NetworkCredential("username", "password", "domainname");
WebService1 objService = new WebService1();
objService.Proxy = objprox;
But I get the exception as "The request failed with HTTP status 407: Proxy Authentication Required." while using the method form webservice.
Can any one suggest for this.
 
...