This error is from exception block.
Yes the form set as full trust before admion approves.
I am using below code to establish connection to webservice before consuming the service.
System.Environment.SetEnvironmentVariable("http.proxyHost", "000.00.00.000");
System.Environment.SetEnvironmentVariable("http.proxyPort", "80");
AuthHeader Credentials = new AuthHeader();
Credentials.Username = "abc";
Credentials.Password = "abc";
objervice.AuthHeaderValue = Credentials;
blnFlag = objService.IsRef_Name(Name.Value);