I will suggest first check is that URL is accessible in browser. If so please check whether it will required secure call, I mean by using Https if so then please add following code
ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(
delegate(object sender2, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
return true;
});