Did you buy a new iPhone and transfer all your information only to find out ‘Hey Siri’ no longer works? This is what happened to my brother and myself after we both purchased new iPhone 12s. We could get Siri to work by holding the Sleep/Wake button in, but if we wanted to use ‘Hey Siri’, both of our new iPhones wouldn’t wake up when we said ‘Hey Siri’. So what do you do to get ‘Hey Siri’ working again? Reset it it. Just open your Settings app, go to Siri & Search, and toggle the Listen for Hey Siri option. Once you turn it off and then turn it back on, your iPhone will ask you to train Siri for your voice. Just follow the steps to train Siri, and then you iPhone will once again be listening for Hey Siri. Again, you’ll find this in the Settings app. Settings>>Siri & Search>>Listen for Hey Siri. Hope it helps!
Update: A member has informed me of another possible fix. Make sure Sound Recognition is off. You will find this in Settings>>Accessibility>> Sound Recognition.