Hello MS1202,
When you want Siri to restart your iPhone, you will need to press the side button to call Siri up and then the rest can be done with your voice, including the confirmation that you want to restart the iPhone. If this is not happening for you, restart the iPhone normally (Restart your iPhone) first just to make sure to clear any issues out that way. Then, test to see if the behavior continues.
If it does, please let us know exactly what you are doing and what responses you get from the iPhone so we can assist further.
Take care.