Answer
USER_MOBILE_PHONE comes from User Properties, not the Person record — RMB on the User page and go to User Properties, then add a MOBILE_PHONE (or USER_MOBILE_PHONE, depending on which the event action actually references) property with the desired value. This is a genuinely separate value from whatever mobile number exists on the underlying Person record, and it's also visible/editable from the Aurena Account Settings screen's User Information section.