Comment by hattar

No; if a phone has both a non-removable battery and a baseband modem, then various laws require that modem to be wired directly to that battery (and to the phone's microphone) and to able to be activated in response to a legal wiretap order, even when the phone itself is nominally "powered off."

(And this doesn't even require that the phone stay connected to the network after the wiretap-enable packet is received. Rather, while the phone is "powered off", the baseband modem might sit there passively acting as a bug, capturing conversation through the microphone onto a bit of NAND onboard the modem; and then, once the phone is powered on again, the baseband modem will take the opportunity to silently play back whatever it's recorded to the tower.)

> if your use case is that sensitive even carrying a smartphone seems questionable.

The issue is that, if you're an actual honest-to-god spy (or investigative journalist...) trying to poke their nose into the goings-on of some government, then you want to draw as little suspicion to yourself as possible; and it's much more suspicious to be going around without the subject government's favorite citizen-surveillance tool on your person. In fact, to blend in, you need to be constantly using your state-surveillance-device to communicate with (decoy) friends and coworkers, doom-scroll, etc.

This is why spies are fans of the few remaining Android phone brands that offer designs with removable batteries. When meeting with a contact, they'll still slip their could-be-bugged-phone into a faraday bag, to cut off its network connectivity; but they'll also remove the phone's battery before putting the phone into the faraday bag, to inhibit this class of "powered-off" record-to-NAND-style baseband wiretap attacks.

(Of course, these are just ways to secure a phone you own + can determine wasn't subject to a supply-chain attack. If two people are meeting who aren't within the same security envelope, then either of them might be trying to surreptitiously record the conversation, and so their phones (or anything else on them) might contain a tiny bug with its own power source, that would stay active even if the macro-scale device's battery was removed. For such meetings, you therefore want to leave all electronic devices in a soundproof safe, in another room. Which will also implicitly act as a faraday cage.)