Error 400 on authentication

I followed the code example at https://developers.ringcentral.com/guide/voice/ringout#c# but getting error 400 on the "Authorize" line.

One or more errors occurred. (Call failed with status code 400 (Bad Request): POST https://platform.devtest.ringcentral.com/restapi/oauth/token).

Here is code:

static RestClient restClient;

static void Main(string[] args)

{

restClient = new RestClient("client-id",

"client-secret",

"https://platform.devtest.ringcentral.com/");

restClient.Authorize(

"+1470502xxxx",

"101",

"XXXXXXX").Wait();

call_ringout().Wait();

}

Can you tell me what did I do wrong?