401 error when trying to retrieve OAuth request token

public static string GetWebRequestPOSTResult(string url, NameValueCollection coll) 
    {
        StringBuilder sbData = new StringBuilder();
        for (int i = 0; i < coll.Count; i++)
        {
            if (sbData.Length > 0)
            {
                sbData.Append("&");
            }
            string key = coll.Keys[i];
            sbData.Append("key=" + HttpUtility.UrlEncode(coll[key]));
        }

        return GetWebRequestPOSTResult(url, sbData.ToString());
    }

    public static string GetWebRequestPOSTResult(string url, string data) 
    {
        string result = string.Empty;

        Uri address = new Uri(url);
        HttpWebRequest req = HttpWebRequest.Create(address) as HttpWebRequest;

        req.ContentType = "application/x-www-form-urlencoded";
        req.Method = "POST";

        byte[] byteData = UTF8Encoding.UTF8.GetBytes(data.ToString());
        req.ContentLength = byteData.Length;

        using (Stream postStream = req.GetRequestStream())
        {
            postStream.Write(byteData, 0, byteData.Length);
        }

        using (HttpWebResponse response = req.GetResponse() as HttpWebResponse)
        {
            StreamReader reader = new StreamReader(response.GetResponseStream());
            result = reader.ReadToEnd();
        }

        return result;
    }

    static void Main()
    {
        DateTime startDate = new DateTime(1970, 1, 1, 0, 0, 0);

        int timeStamp = (int)((DateTime.Now - startDate).TotalSeconds + 601);
        string nonce = Guid.NewGuid().ToString().Replace("-", "").Substring(0, 8);

        System.Collections.Specialized.NameValueCollection appSettings = System.Configuration.ConfigurationManager.AppSettings;

        string secretKey = appSettings["ydn_sharedsecret"];
        string apiKey = appSettings["ydn_apikey"];
        string appId = appSettings["ydn_appid"];

        NameValueCollection coll = new NameValueCollection();
        coll.Add("oauth_nonce", nonce);
        coll.Add("oauth_timestamp", timeStamp.ToString());
        coll.Add("oauth_consumer_key", apiKey);
        coll.Add("oauth_signature_method", "HMAC-SHA1");
        coll.Add("oauth_version", "1.0");
        coll.Add("xoauth_lang_pref", "en-us");
        coll.Add("oauth_callback", "http://test.desjalogic.com/YahooContactsHandler.aspx");
     
        string result = GetWebRequestPOSTResult("https://api.login.yahoo.com/oauth/v2/get_request_token", coll);
        Console.WriteLine(result);
    }