HubSpot has a very detailed article in their knowledge base if you want more details.
Here is an excerpt.
“The HubSpot calling tool allows you to call contacts from your browser or phone and log calls on your contacts’ timelines in HubSpot.
The number of calling minutes in your HubSpot account depends on your subscription. An alert will appear in your account when you’re within 100 minutes of your limit. If you reach your limit while on a call, you will be able to complete that call, but will not be able to make additional calls that month. Calling minutes reset on the first day of each month.