Which TV and phone?
Sony has there own remote app for Android and I'm pretty sure it's on iOS too. It can turn on the TV.
Oops, didn't notice you said "via voice", sorry.
CetusPlay offers this function on Fire TV and Google TV. I am not sure about your devices. But you can try it. It is free. So no costs to try.
Yep. There's probably a few ways to accomplish this, but the way I do it is a combination of the Harmony hub+ app, Tasker, Autovoice, and Harmony Tasker plugin.
Basically you install the Harmony app, Tasker and both plugins. Then you set Autovoice as the trigger using whatever phrase you want to set up, and then set the action to a harmony hub command or activity.