The remote server returned an error: (406) Not Acceptable

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C#

in my code i have 3 urls.when i executing the values are passed in the urls..after some time got an error .

The remote server returned an error: (406) Not Acceptable..

my code

foreach (Eben_Longcode_Manager_V3.Classes.ShortMessage msg in objShortMessageCollection)
                {
                    msg1 = msg.Message.Replace("'", " ");
                    string st = msg1.Substring(msg1.Length - 2, 2);
                    if (st == "OK")
                        msg1 = (msg1.Remove(msg1.Length - 3)).Trim();
                    msg1 = System.Web.HttpUtility.UrlEncode(msg1.ToString());
                    //string query = "INSERT INTO long_msg(num,msg,date) VALUES('" + msg.Sender.ToString() + "', '" + msg1 + "','" + DateTime.Now.ToString("MM/dd/yyyy").ToString() + "')";
                    //MySqlCommand cmd = new MySqlCommand(query, connection);
                    //cmd.ExecuteNonQuery();                        
                    string no = msg.Sender.ToString().Replace("+", "");
                    string url = "http://traffic.ebentelecom.co.in/WldKbGJuUnlZV1ptYVdNPQ.php?msg=" + msg1 + "&mob=" + no + "&route=Longcode&sim=" + mob;
                    string url1 = "http://www.trafficmanager.in/WldKbGJuUnlZV1ptYVdNPQ.php?msg=" + msg1 + "&mob=" + no + "&route=Longcode&sim=" + mob;
                    string url2 = "http://airtel.ebentelecom.com/airtel_longcode.php?mobile=" + no + "&message=" + msg1;
                    using (WebClient client = new WebClient())
                        client.DownloadString(url);
                    using (WebClient client1 = new WebClient())
                        client1.DownloadString(url1);
                    using (WebClient client2= new WebClient())
                        client2.DownloadString(url2);
                    System.Threading.Thread.Sleep(500);
                    msg_read = msg_read + 1;