Calling Soap web service over HTTP giving exception "Connection close"

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Xml;

namespace CTWpfApp
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            CallWebService();
        }
        public static void CallWebService()
        {
            var _url = "https://myService.com/webservices/ct/services/4.1";
            var _action = "urn:provider/interface/ctservices/getCPNInstances";

            XmlDocument soapEnvelopeXml = CreateSoapEnvelope();
            HttpWebRequest webRequest = CreateWebRequest(_url, _action);
            InsertSoapEnvelopeIntoWebRequest(soapEnvelopeXml, webRequest);

            // Start the asynchronous operation to get the response
            webRequest.BeginGetResponse(new AsyncCallback(GetResponseCallback), webRequest);
        }

        private static void GetResponseCallback(IAsyncResult asynchronousResult)
        {
            HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;

            // End the operation /* I'm getting the exception Connection Close."*/
            HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult); 
            Stream streamResponse = response.GetResponseStream();
            StreamReader streamRead = new StreamReader(streamResponse);
            string responseString = streamRead.ReadToEnd();
            Console.WriteLine(responseString);
            // Close the stream object
            streamResponse.Close();
            streamRead.Close();

            // Release the HttpWebResponse
            response.Close();           
        }

        private static HttpWebRequest CreateWebRequest(string url, string action)
        {
            HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
            webRequest.Headers.Add("SOAPAction", action);
            webRequest.ContentType = "text/xml;charset=\"utf-8\"";
            webRequest.Accept = "text/xml";
            webRequest.Method = "POST";
            webRequest.KeepAlive = false;
            webRequest.Timeout = 300000;
            return webRequest;
        }

        private static XmlDocument CreateSoapEnvelope()
        {
            XmlDocument soapEnvelop = new XmlDocument();
            soapEnvelop.LoadXml(@"<SOAP-ENV:Envelope xmlns:SOAP-ENV=""http://schemas.xmlsoap.org/soap/envelope/"" xmlns:ns1=""urn:dictionary:com.ct.webservices""><SOAP-ENV:Header xmlns:wsse=""http://docs.myses.org/wss/2004/01/200401-wss-wssecurity-secext-1.0.xsd""><wsse:Security SOAP-ENV:mustUnderstand=""1""><wsse:UsernameToken><wsse:Username>fiwjiueji</wsse:Username><wsse:Password Type=""http://docs.myses.org/wss/2004/01/200401-wss-username-token-profile-1.0#PasswordText"">tjrrfrsi</wsse:Password></wsse:UsernameToken></wsse:Security></SOAP-ENV:Header><SOAP-ENV:Body><ns1:getCPNInstances></ns1:getCPNInstances></SOAP-ENV:Body></SOAP-ENV:Envelope>");
            return soapEnvelop;
        }

        private static void InsertSoapEnvelopeIntoWebRequest(XmlDocument soapEnvelopeXml, HttpWebRequest webRequest)
        {
            using (Stream stream = webRequest.GetRequestStream())
            {
                soapEnvelopeXml.Save(stream);
            }
        }
    }
}