I've turned my microphone and camera off in the BIOS. Pretty sure it can't be activated unless allowed in the BIOS. But I've searched and not found a definite answer. You would have to go through a boot cycle to reinitialize the BIOS with new settings. And if my computer did an unrequested boot I'd be suspicious. I check on it every once in a while to make sure the settings are off. I've checked with online sites that activate the camera and microphone to test them and they say those devices do not exist on my system. So I'm pretty confident they are non accessible from the outside world.
My phone on the other hand is on 24/7. And even if not on it is still powered and I've been told the microphone can be activated even with the phone in the off condition. I can no longer remove my battery so it can never be off off off. Best you can do is put it into a Faraday bag and put it into a box.