Android: You can do SmartThings voice commands on Android with SharpTools:
http://sharptools.boshdirect.com/examples/autovoice-control-device
iOS: There was also a thread about doing this with Siri, but the voice commands didn’t seem very natural to me.
Amazon Echo: Toward the bottom of the Amazon Echo thread, there was also some discussion about a user who is reverse engineering the system and has voice commands working.