Currently to log calls automatically into the HubSpot CRM we require a contact to be associated with the 10 digit phone number. If a contact is not manually assigned for the call record - RingCentral will create one for you by appending the type of communication (Call or SMS) and the 10 digit phone number as the first and last name respectively. Later users have the way of modifying that contact by either appending the right information or merging it with an existing contact in HubSpot. 

There are plans to address the contact creation settings later in the year - but for now if it remains to be large impact we recommend setting up a workflow to delete these “Unknown Contacts” within HubSpot.