You have a custom entity named Event Registration. Event Registration has a relationship with the contact entity. You plan to add a field named Contact Phone to Event Registration. You need to…

You have a custom entity named Event Registration. Event Registration has a relationship with the contact entity. You plan to add a field named Contact Phone to Event Registration. You need to ensure that when a contact is added to an Event Registration record, the phone number of the contact is displayed in the Contact Phone field of the record. If the contact changes, the Contact Phone field must display the new phone number of the contact. What should you use?