possible – but not exactly a trivial task.
This would require physical access to the target telephone equipment (to implant or accept the rogue software updates) or for the target to have their phone subsituted with a similar model.
And there is no way that equipment that has been electrically switched off (or had its battery removed) could be activated in such a way.
I expect the technology is used in corporate surveillance or by estranged partners spying on each other already; and it might work on some gangsta rapper/dealer type too dumb or arrogant to be properly surveillance-aware but its not quite tinfoil helmet material IMO…
similar things on the fixed telephone network have existed for many years.
They were called “infinity bugs” and were far more commonly used to listen in on activists etc back in the 1970s and 1980s when most telephones were hard-wired to a network under the control of a single company or public telecoms authority. Old dial phones had plenty of space inside to hide extra components…
