You can use Siri with the headset that came with iPhone, and with other compatible wired or Bluetooth headsets.
Talk to Siri using a headset: Press and hold the center button (or the call button on a Bluetooth headset).
To continue a conversation with Siri, press and hold the button each time you want to talk.
When you use a headset, Siri speaks its responses to you. Siri reads back text messages and email messages that you’ve dictated before sending them. This gives you a chance to change the message if you want. Siri also reads back the subjects of reminders before creating them.