Microsoft Cortana is now available for iPhone, Android and even cyanogen OS. Cortana is an answer to Apple Siri but microsoft says, it's the first digital personal assistant that is designed to answer your voice commands or requests and more( Like cortana can tell you jokes, sports results, set appointment on calendar, change your appointments, tell you the nutrition fact of food and also can sing a song). Even you can activate Cortana by just saying "Hey Cortana" but this option is currently available in Windows Phone and to do that in your Windows Phone, you have to open the Notebook settings and then turn on the Let Cortana respond when you say "Hey Cortana"