If Person Accounts (only addresses as contact but there is an account ID and contact ID per address) is enabled in Salesforce, an additional logic for call assignment is used
•if an outbound phone call starts from the account object it is assigned to the contact object
•for any situation where an account is selected manually or automatically it is reassigned to the contact object
This allows assigning a Related To object like case or opportunity to each phone call
See configuration for Person Accounts in the SoftPhone Layout