Even current smartphones do not turn on their microphones outside of a call, unless one has made the huge mistake to turn their assistants on, or there's an app recording - and at least on iOS that's controllable by permissions and marked clearly when the app is in the background.
Anyway, smartphones are creepy too.
These days, I'm not sure at all. All we have is the law as a deterrent, and that's weak, especially to governments.
One cannot know with absolute certainty, only with reasonable certainty.
On the other hand, an assistant is listening all the time by design.
If one needs to know with absolute certainty, then they cannot use a mobile phone, nor probably a reasonably modern regular phone.
ISTR there are documented instances of law enforcement and/or intelligence doing that sort of thing to phones, and the battery drain causing it to get noticed.
Hooking it up to an outlet is a legit different thing.
If battery tech or power reduction tech progresses sufficiently, including just by continuing on its current curve (most power reduction rather than battery tech), cell phones will rise to the same level of concerning.
(Cell phones of course leak location data full time, and some similar things like that. I'm not saying the current cell situation is awesome either. Only that there is a legit reason to be more concerned about gear hooked up to actual power lines than portable gear.)
We know that modern phones fan constantly listen to the microphone with minimal power impact, since that’s what these voice assistants are doing. That’s specialized hardware, but adding a little something to save the data wouldn’t be hard.
Even if we take it as a given that it can’t be done on batter power, phones get connected to outlets pretty regularly. Have you ever heard someone refuse to charge their phone in their house because it’s creepy?
Google's actual smartphone line (Pixel) ships with passive song recognition, default disabled. I enabled it because I was curious and it sips so little power that I left it on, but to function it has to be listening to the audio all the time so that it can say to itself "Hey, that is Sabotage by the Beastie Boys".
It doesn't use the network, or the screen (unless you power the screen on to find out what you're listening to) but it passively remembers every recognisable song it heard, essentially forever. So that means it is _listening_ it just doesn't tell anybody about it.
We have not historically been there, which informs people's about cell phones, and we are not quite there yet. 4 or 5 years maybe.
(Although I have some interesting thoughts on how to deal with that. It would be fun to take one of these "transcribing phones" into a movie theatre, then fire the MPAA at Google for copyright infringement.)
Does it in some sense "wake up" to do this? Yes, I assume so.
I don't know that an old-school land line phone isn't always listening. But it was made by some disinterested party who just makes hardware. It was a simple logical device designed to rely on a simple mechanical switch. It could be listening, but it's understandable by someone like me and there's no incentive for anyone involved. I don't really know if I'm in control, but it's reasonable to think I am.
I don't know that a modern voice assistant isn't always listening either. It is an incredibly complex device usually designed by an entity whose business makes hundreds of millions of dollars based off of the inspection of the personal data of people like me. It is a complex full functioning computer. It uses a statistical algorithm that I know will incorrectly activate sometimes. I cannot understand or predict when it will listen. I cannot understand or predict when it will send a recording somewhere. It receives updates frequently. Because it's a full computer with an internet connection, those updates could make it do a huge variety of things. I know I'm not in control.