some error sending sms please help me in asp.net C#

private void send_sms_direct(string numbers, string sms)
    {
        if (txtmobile_direct.Text == string.Empty)
        {
            Response.Write("Please Enter Numbers");
            return;
        }

        if (txtMessage.Text == string.Empty)
        {
            Response.Write("Please Enter Your Message");
            return;
        }

        string authKey = "70740A9HJVjbP53e5cee9";
        string mobileNumber = numbers;
        string senderId = "NBCHOS";
        string message = HttpUtility.UrlEncode(sms);
        StringBuilder sbPostData = new StringBuilder();
        sbPostData.AppendFormat("authkey={0}", authKey);
        sbPostData.AppendFormat("&mobiles={0}", mobileNumber);
        sbPostData.AppendFormat("&message={0}", message);
        sbPostData.AppendFormat("&sender={0}", senderId);
        sbPostData.AppendFormat("&route={0}", "4");

        try
        {
            string sendSMSUri = "https://control.msg91.com/sendhttp.php";
            HttpWebRequest httpWReq = (HttpWebRequest)WebRequest.Create(sendSMSUri);
            UTF8Encoding encoding = new UTF8Encoding();
            byte[] data = encoding.GetBytes(sbPostData.ToString());
            httpWReq.Method = "POST";
            httpWReq.ContentType = "application/x-www-form-urlencoded";
            httpWReq.ContentLength = data.Length;
            using (Stream stream = httpWReq.GetRequestStream())
            {
                stream.Write(data, 0, data.Length);
            }
            HttpWebResponse response = (HttpWebResponse)httpWReq.GetResponse();
            StreamReader reader = new StreamReader(response.GetResponseStream());
            string responseString = reader.ReadToEnd();
            reader.Close();
            response.Close();
        }
        catch (SystemException ex)
        {
            Response.Write(ex.Message.ToString());
        }

       
    }