The fastest method of activating Siri on iPhone 4s, iPhone 5c, or iPhone 6 is to press and hold down the Home button on the device. Doing so prompts Siri to ask, “What can I help you with?”
An alternate method enables Siri’s hands-free use and requires tapping Settings, then General, then Siri, and then sliding Siri’s toggle switch to the "on" position, which turns the switch green. Users slide the Allow “Hey Siri” toggle switch "on" to finish enabling Siri.
Once enabled, the feature lets users command Siri hands free by just saying, “Hey Siri.” After enabling Siri, users can change its language, voice gender, voice feedback, and the names that Siri calls them by using the menu beneath the Siri toggle switch.