string lcUrl = "http://www.urldelsito.com/pagina.php";

            HttpWebRequest loHttp = (HttpWebRequest)WebRequest.Create(lcUrl);

            // *** Send any POST data
            string lcPostData =
                "telefono" + HttpUtility.UrlEncode("064616948321");

            loHttp.Method = "POST";
            byte[] lbPostBuffer = System.Text.Encoding.GetEncoding(1252).GetBytes(lcPostData);
            loHttp.ContentLength = lbPostBuffer.Length;

            Stream loPostData = loHttp.GetRequestStream();
            loPostData.Write(lbPostBuffer, 0, lbPostBuffer.Length);
            loPostData.Close();

            HttpWebResponse loWebResponse = (HttpWebResponse)loHttp.GetResponse();
            Encoding enc = System.Text.Encoding.GetEncoding(1252);
            StreamReader loResponseStream = new StreamReader(loWebResponse.GetResponseStream(), enc);

            string lcHtml = loResponseStream.ReadToEnd();

            testo.Text = lcHtml;

            loWebResponse.Close();
            loResponseStream.Close();