Short answer
HubSpot's calling FAQ: carriers may block calls because of the caller ID; HubSpot links to registering numbers with carriers' analytics providers. It recommends permission-based calling and, if your usual number is consistently labeled as spam, keeping a HubSpot-provided number as a fallback.
1. Numbers
Call setup in my demo portal.

2. Blocked numbers
To stop calls to specific numbers, see blocked numbers.