Short answer
HubSpot's KB: providers build inbound support with the Calling Extensions SDK. When you use it, the app opens in a separate calling window, and that window must stay open to make and receive calls. You may need to log in to your provider there.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Not every calling app supports inbound in HubSpot. HubSpot-provided numbers handle inbound calls without a third-party app.
3. Related
HubSpot calling numbers and connecting a calling provider.