curl -X POST "https://platform.ringcentral.com/restapi/oauth/token" 
-H "Accept: application/json"
-H "Content-Type: application/x-www-form-urlencoded"  -H "Authorization: Basic <access-token>"  -d "username=<user-phone-number>&password=<user-password>&extension=<user-extension>&grant_type=password   

Here the problem you are facing is the header Authorization which is Basic <base64 of appKey:appSecret> .. If you pass that header in curl, your problem will be solved