I'm trying to create a server-only (no UI) application with the RingCentral API. The goal is to use their new address book endpoint to automatically create/update the user contacts for all extensions in our company.
This seems like it should be straightforward and shouldn't require user authentication (since it's a private application), but I can't see from the docs how I can authorize such an API call. The auth options seem to all require the user to provide explicit authorization. Is there some way to do what I'm trying to do with just the client access id and secret provided by RC?
The RingCentral API still requires OAuth 2.0 user authorization for server-only private applications because the authorized user is used to determine authorization scope. For example, a user with administrative privileges can access/modify resources on a company-wide level while an individual user may only be able to access/modify resources for their own user.
In your case, if you want to create a private app (private = for use only by your company), a simple way is to use the Resource Owner Password Credentials grant (username, extension, and password) in your code without requiring a UI (OAuth pop-up).
If you want this app to have administrative access you can use the credentials for a user extension with admin privileges. You can either use the existing admin extension or create a new one and assign admin privileges.