Proxy Authentication require error...

Hi when I run this routine:

 private void VoicepayCCProcess(string sRecordline, string sRequeststring)
        {
            try
            {
                string result;

                HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://secure.voice-pay.com/gateway/remote_auth");
                request.ContentType = "application/x-www-form-urlencoded";
                request.Method = "POST";

                using (StreamWriter writer = new StreamWriter(request.GetRequestStream()))
                {
                    writer.Write(sRequeststring);
                }
               
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                using (StreamReader reader = new StreamReader(response.GetResponseStream()))
                {
                    result = reader.ReadToEnd();
                }

                // Once we get response back from the voicepay we can construct the output file line by line.
                WriteToFile(sRecordline, result);
            }
            catch (WebException ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

On this line:  using (StreamWriter writer = new StreamWriter(request.GetRequestStream()))

I'm getting this error:
the remote server returned an error 407 proxy authentication required

Can you  help me please.