Please read this https://medium.com/ringcentral-developers/a-minimal-project-to-implement-ringcentral-auth-code-flow-in-javascript-bf995d458424
It's not C# but it's minimal. It's good for you to understand the whole flow. It's a single JS page, just like you want to do it in a single controller.
So in the controller code, you need to check wether there is query parameter named "code".
If yes, you exchange the code for access token and do whatever you want (send sms).
If no, you need to redirect user to the login URI, which can be generated using https://github.com/ringcentral/RingCentral.Net/tree/master/RingCentral.Net.AuthorizeUri
Be sure to set the redirect URI to the controller page, so that after user logged in, he will redirected back to you controller with "code" in query parameter.
