Re: yet another reason...
Depends on a variety of things :)
Your phone mobile is permanently wired, it can't be disconnected. As to wether it's "active" is down to software, listening to the input channel.
The various assistants (Siri etc) can be configured to either "listen all the time" (keyword activation) or after a button press (Siri). But do you trust that's what they're actually doing? :)
If you've got a Smartphone, and you can't take the battery out, you can't be 100% certain that it's not listening, That's the basic fact. Assuming you haven't run some third party apps with access to microphones, or granted access to those apps, then you should be "as safe as your assistant settings" are configured.