Hi Keith,
When you create a RingCentral app from your developer account, if you don't have a sandbox setup, the system will automatically create a sandbox for you with a company main number. You will then receive an email (to the email inbox you used to signup with RingCentral) with the activation information. You must activate your sandbox account and set the password for it. After that, you can get back to your developer account and copy the sandbox credentials (sandbox server url, app key and app secret), keep them in a safe place so you can use them later in your code.
When you create your app, you can choose the platform type based on how you let the user to login and to get authenticated, and how you will call RingCentral APIs (call the APIs from client code or from server code).
If you don't require a user to login your RingCentral account (always login with your designated credentials) to be able to use your app, you can simply choose the "Server-only (No UI)" type. Then you can call RingCentral APIs from the backend code.
If you let your users to login with their own credentials (let's say you have 10 employees and each with their own number), then choose the "Server/Web" if you want to call RC APIs from the backend code or choose "Browser" if you want to call RC APIs from the client code.
Based on what you said above, I think you can simply choose the "Server-only (No UI)" for the platform type. Then implement code to login and to call RingCentral APIs from the backend.
Hope this helps.
+ Phong
