Subscribe to Events Broadcast from another Server

Question

I am new to .NET Core and have been tasked with creating a phone application that will allow our employees to see their call queues, view information about the customers that are calling, etc. Our phone provider offers an API that allow us to subscribe to events via HTTP (COMET). The structure I am building is as follows:

1. Phone Service Broadcasts Event from their servers
2. Our backend sees that event & gathers some additional info from our systems about the caller
3. Our backend sends that data to our frontend application.

Our employees would have this application open and when one of our customers calls in, the frontend will update in real time with the customer's information.

I am stuck on how to perform #2 in .NET Core. I have heard of gRPC, SignalR and System.Threading.Channels, but every example I see has a user triggering an event and the backend then broadcasts that change. We do not have users triggering events, we need to respond/subscribe to events in real time from another server. I am wondering what I should be using in .NET to accomplish this.