Go to Settings > General > Siri, where you can:
Turn Siri on or off
Turn “Hey Siri” on or off
Specify the language to use
Set voice accent and gender (may not be available in all areas)
Specify your My Info card
Choose a setting for Voice feedback
To allow Siri to speak when the Ring/Silent switch is set to silent mode, choose Always On
To allow Siri to speak only when the Ring/Silent switch is set to ring mode, choose Control with Ring Switch.
To allow Siri to speak only when you’re using “Hey Siri” or when iPhone is connected to a Bluetooth device, headphones, or CarPlay, choose Hands-Free Only. (When the Hands-Free Only option is selected, Siri also reads back information.)
Prevent access to Siri when iPhone is locked. Go to Settings > Touch ID & Passcode (iPhone models with Touch ID) or Settings > Passcode (other models). You can also disable Siri by turning on restrictions.