The remote server returned an error: (401) Unauthorized

Using visual studio 2015, C#, constructed code to send sms using Ring Central SMS api,

Error details :- "The remote server returned an error: (401) Unauthorized."

Error while running the below code:

var JSONstr = new StringBuilder();

System.Net.WebRequest req = null;

System.Net.WebResponse rsp = null;

System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Ssl3 | System.Net.SecurityProtocolType.Tls11 | System.Net.SecurityProtocolType.Tls12;

JSONstr.Append("{");

JSONstr.Append(""to"" + ":" + " [{" + ""phoneNumber"" + ":" +""+1234567890"" + "}]" + ",");

JSONstr.Append(""from"" + ":" + " [{" + ""phoneNumber"" + ":" + ""+12587419632"" + "}]" + ",");

JSONstr.Append(""text"" + ":" + ""Test SMS message"");

JSONstr.Append("}");

string uri = "https://platform.ringcentral.com/restapi/v1.0/account/+19294682441/extension/101/sms";

if (uri != string.Empty && uri != null)

{

req = System.Net.WebRequest.Create(uri);

req.Method = "POST";

req.ContentType = "application/json";

System.IO.StreamWriter writer = new System.IO.StreamWriter(req.GetRequestStream());

writer.WriteLine(JSONstr.ToString());

writer.Close();

rsp = req.GetResponse();

Kindly help me..