Invalid Resource Owner Credentials

Here's the flow of my application. First, I made an application in my sandbox account and set the type as Server-Only (no UI). Then i used the client_id, client_Secret, unsername(it's the main company phone number mentioned in my account), server_url and extension_id. I used password with whicg i logged in in my ringcentral account but still I'm getting this error that "invalid resource owner credentials.". Can you please guide me how to get rid of this error. thanks