I don't know of any android phones that would allow this by default, but if you have a samsung galaxy phone and a Gear 2 Neo, you can use S-Voice via the smartwatch without touching your phone...
From another forum... again for a Samsung phone...
"go into the S Voice app and select "Settings" and then "Open via the home key". I also selected "Voice wake-up" and "Wake up command". In other words the only way to do this is to wake wake up the Note 3 while the screen is off with "Hi, Galaxy" and go from there."
Not tried this myself.
Edit: found this app on google play...
https://play.google.com/store/apps/details?id=cx.makaveli.voicelockscreenhd