As hinted earlier this summer by Marcus Ash from Cortana team, passive voice activation is coming to Windows Phone to catch up with Android and iOS. Arriving later this fall with the newly announced 'Lumia Denim' firmware update, some Lumias are getting the hands-free feature named 'Hey Cortana.' Users with smartphones featuring the Snapdragon 80x chipsets, like the Lumia 930, Lumia Icon, and Lumia 1520 can all take advantage of the Cortana extension.
After a brief training session whereby the phone learns your specific voice, users will be able to call up Cortana by simply saying 'Hey, Cortana' (or any other name they bequeath to the voice-assistant). Cortana then wakes up and takes your inquiry. Such a feature allows users to use Cortana without ever touching their phone, which is perfect if you are across the room, have your hands full, are driving or other situations where it is more convenient to go hands-free.
Presumably other phones like HTC's new One M8 for Windows Phone can do the same, as that phone has the necessary hardware, but it also needs a firmware/OS update to enable the new feature. For Microsoft and their Lumia line, 'Hey Cortana' is supposed to launch with the new Lumia Denim firmware and OS update expected later this fall.