How do acquire the phone number of an incoming call?

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'.

Is there a example to achieve this through asp.net