Most Alexa-powered devices can be controlled entirely with voice commands right out of the box by design—just say “Alexa” and your Echo speaker, Fire TV or other Alexa device will respond, no button-presses or taps necessary. The rare exception is the Alexa smartphone app, which requires users tap an “Alexa” button…
Source: LifeHacker – How to Use the Alexa App Hands-Free