JTAPI might be a bit heavy for your ASP.NET application if all you need is the calling number. You can do this and stay entirely web based.
I would recommend you look at the Cisco CURRI API. It's a XML formatted HTTP interface that fires an event on every call received on an extension, route pattern, or translation pattern. Really simple to configure and develop with, and since it's web based, you will be able to natively receive it from within your ASP.NET application.
Cisco Docs - with full examples of requests and responses
https://developer.cisco.com/site/curri/documents/latest-version/
You just need to develop 2 things at a minimum:
1. A keep-alive flow - This flow tells CUCM your CURRI endpoint ready for events. This is outlined in the above docs.
2. A request/response flow - When CURRI sends you a request event you would return a simple continue policy response message (see the example in the docs), and just take the phone number from the XML request for your own application usage.
From there, in your own app, you might want to build status indicators to know if you the keepalive is healthy, and alerts when it goes down, etc.
If you don't want to develop your own CURRI or JTAPI integration, and want something pre-made for CURRI that can trigger customizable webhooks with CURRI realtime call data, I have a product that does this - https://calltelemetry.com
-Jason
