This is largely going to depend upon "when" you need the data, but to broadly answer your question, you can use either the
active-calls or
presence for a particular extension.
If you need the data in near-real-time, I would recommend creating a Subscription (Push Notification) or Webhook (also Push Notification) for the Extension(s) you wish to receive this information. Here is a
sample application showing how to use RingCentral Subscriptions. Here is a
sample application showing how to use RingCentral Webhooks. Here is the
documentation for Push Notifications.
If you need the incoming caller's phone number you can also use the active-calls or presence as I listed initially, but the rate limiting on these may restrict your usage of these resources which is why I suggested Subscriptions or Webhooks.
If you need this for analysis, I would recommend using
Call-Log, and set the query parameter 'view=Detailed'.
