using (System.Net.WebClient client = new System.Net.WebClient())
{
    try
    {
        String url = "http://smsc.vianett.no/v3/send.ashx?" +
            "src=" + phone.Text + "&" +
            "dst=" + phone.Text + "&" +
            "msg=" + System.Web.HttpUtility.UrlEncode(messagebody.Text,System.Text.Encoding.GetEncoding("ISO-8859-1")) + "&" +                        "username="+System.Web.HttpUtility.UrlEncode(username.Text) + "&" +
            "password="+System.Web.HttpUtility.UrlEncode(pass.Text);
        String result = client.DownloadString(url);
        if (result.Contains("OK"))
        {
            MessageBox.Show("message sended");
        }
        else
        {
            MessageBox.Show("message not sended");
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}