Tit is correct as long as they know the IMEI of a SIM card they can pretty do whatever they like with a phone as long as it's got a battery in it (with power obv.). There are a lot of people who worry about shit like this but the truth is unless they have a reason to monitor you then they won't.
One of the coolest things I saw in a previous life was a Bluetooth rifle. At a range of 100-200 yds it could latch on to a phone conversation.
there really is though, that's the issue. the system is so corrupted and biased that giving this much power to people so incapable of using it 'well' is going to lead us to bigger and bigger problems.
we're even now seeing computer manufacturers themselves installing abhorrent spyware you'd be trying to get rid of for ages if you accidentally installed it yourself yet then claiming it's for the good of the customer when found out. The US is working to undermine the security of everything we do on the basis that they will know best whilst at the same putting us in a worse and worse position vs those who wish any kind of harm done ie china, bad people, criminals whatever word you use to describe the things that you fear.
the argument that if you're not doing anything wrong (remembering wrong is defined by the people who are spying on you) means you have nothing to fear, is utterly two faced and undefendable. watch this for a better presented reasoning.