Short answer
HubSpot's KB: accounts have a pooled limit of numbers by subscription. Only Super Admins acquire or reassign them. They don't accept calls from private or anonymous numbers and can't be used for Hosted SMS with other providers.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Check your remaining numbers before promising one to a team.
3. Related
Number types and calling numbers.