This is a useful feature and you don't need to say the exact phrase for it to work.
To answer the call on speaker, you can just say "answer"
To dismiss an alarm, you can just say "stop" or "dismiss"
But I noticed that sometimes, it doesn't seem to be available.
So I have to wake-up Bixby.
Going back to the settings, it says the feature will not work while connected to a Bluetooth device.
Well, I wasn't connected to a Bluetooth device but it wasn't working even if I have the feature switched on.
Then I figured it was working while I was outside the house even at nighttime. So maybe do not disturb is preventing it to work.
And I was right. I tried switching off DND and it worked. Ahahahaha.
