Just as most other voice assistants, Siri is essentially always on, through your HomePod or iPhone microphone, listening for you to say “Hey Siri” so it can get activated. According to Apple privacy policy, although Siri is listening, it doesn’t remember anything you say before it’s activated by its voice command.
When you call a dead phone does it ring?
Answer: A: With a dead battery it should not ring but it should go to voicemail directly.