I can't get the "Hey Siri" feature activated on my phone so I can change volumes on AirPods when using

I can't get the "Hey Siri" feature activated on my phone so I can change volume on Airpods when using. It's not reading/hearing my voice when I try to set up "Hey Siri" feature. Wondering if a feature only for new Iphone models. I have an old Iphone 6 plus.

[Re-Titled by Moderator]