Of all the standard fields available in the Event and Task object, two are only useable to display in detail view:
They are pulling basic information from the related Contact or Lead (WhoId) and make it easy to pass a phone call without any additional click!
The problem starts when you want to access this data through a formula, validation rule, field update, process builder, API... They aren't exposed.
Please expose these fields to the API and declarative business logic components.