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{    
  "error" : "unauthorized_client",    
  "error_description" : "Unauthorized for this grant type",    
  "errors" : [ {      
    "errorCode" : "OAU-251", 
    "message" : "Unauthorized for this grant type"    
  } ] 
}

Here is my code below:-

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://platform.devtest.ringcentral.com/restapi/oauth/token");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, "username=+12048184079&password=Reset321$$&extension=101&grant_type=password");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_USERPWD, "1RjNgM-LTxxxxxFuE-ggrHA" . ":" . "524fjcxxxxxxxxxxCPclIZHxxxxxx_FdfegzA");

$headers = array();
$headers[] = "Accept: application/json";
$headers[] = "Content-Type: application/x-www-form-urlencoded";
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

$result = curl_exec($ch);

echo '<pre>'; print_r($result); die;

if (curl_errno($ch)) {

echo 'Error:' . curl_error($ch);
}

curl_close ($ch);