To talk to Siri, you can hold down the Home button and speak, or raise the phone to your ear and speak. If you prefer the latter, you should make sure the setting named Raise to Speak has been turned on. For your better understanding, this article respectively illustrates how to turn it on and off in iPhone or iPad.
Step 1: Get into Settings > General > Siri.
Step 2: Click the switch beside Raise to Speak to turn it on.
Enter Settings > General > Siri, and then tap the switch on the right of Raise to Speak to turn it off.