For those on iOS 9.1 beta, go to Settings > General > Siri to enable the 'Allow "Hey Siri" feature. This cues the setup for voice recognition, which has you repeating "Hey Siri" several times along with two additional test phrases. Once done, Siri's all yours. Not seen in Siri's settings: an option for always on "Hey Siri" that was discussed in Wednesday's keynote. I'm assuming that's 6s/ 6s Plus only?