Hi Tnavgun,
We recommend that you try the steps in the link below since "Hey Siri" isn't working as expected on your iPhone.
If "Hey Siri" isn't working on your iPhone - Apple Support
To perform the steps in the article we suggested above, click and open the link, read the subheadings below, and try each step.
1) Check if "Hey Siri" is turned on
1. Go to Settings.
2. Tap Siri & Search.
3. From here, make sure the following settings are turned on:
* Listen for "Hey Siri"
* Allow Siri When Locked
2) Set up Siri to recognize your voice
1. Go to Settings.
2. Tap Siri & Search.
3. Turn off Listen for "Hey Siri," then turn it back on.
4. When the Set Up "Hey Siri" screen appears, tap Continue.
5. Say each command that you see on your screen.
6. Tap Done.
3) If Siri stops responding
1. Go to Settings.
2. Tap Siri & Search.
3. Tap Siri Responses.
4. Tap Prefer Spoken Responses to hear Siri speak responses, even when Silent mode is turned on.
The reason why we are recommending the steps above is because they will show you what to do if Siri isn't working as expected on your iPhone.
Please let us know the outcome after giving the above steps a try.
Thanks for using the Apple Support Communities.
Cheers!