Final answer:
The nurse manager should activate the agency disaster plan in response to the hurricane warning.
Step-by-step explanation:
The nurse manager who received the telephone call regarding the hurricane warning should take the initial action of activating the agency disaster plan. This is because a hurricane is a natural disaster that can potentially cause widespread damage and casualties. By activating the agency disaster plan, the hospital can ensure that they have procedures in place to effectively respond to the event and provide appropriate care for any casualties that occur.