Short answer
HubSpot's KB: HubSpot-provided numbers are powered by Twilio and are the only ones that receive inbound calls to shared channels like help desk. Registered outbound numbers are existing lines; each rep verifies their own by SMS or call code, and admins can't assign them.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Pick by whether you need inbound calls to a team.
3. Related
HubSpot calling numbers and blocked numbers.