There is a maintenance mode in every cell phone that allows it to be remotely turned on, that is, used as a listening device, without your knowledge.
I don't know what authentication is required. I expect that it was designed so that only your cell carrier could enable it, however whatever may have been secret about it, quite likely has leaked out by now.
If you don't want to be listened-to, don't have _any_ cell phones anywhere near you. Not just your own - say you want a private conversation in a public place; the phones of other people in your general vicinity could be switched on to listen to you.
I learned this from a well-known left-wing radical organization known as the United States Air Force, when I applied for the USAF Cyber Command. Their site had a recruiting video, that depicted a couple officers locking their phones into a grounded metal box - a faraday cage - before entering a secure area, that is, a room where secrets were openly discussed.
I don't know what authentication is required. I expect that it was designed so that only your cell carrier could enable it, however whatever may have been secret about it, quite likely has leaked out by now.
If you don't want to be listened-to, don't have _any_ cell phones anywhere near you. Not just your own - say you want a private conversation in a public place; the phones of other people in your general vicinity could be switched on to listen to you.
I learned this from a well-known left-wing radical organization known as the United States Air Force, when I applied for the USAF Cyber Command. Their site had a recruiting video, that depicted a couple officers locking their phones into a grounded metal box - a faraday cage - before entering a secure area, that is, a room where secrets were openly discussed.