How can i know if my phone is being eavesdroped on through the cellular network?

Friend of an ex-friend can do this. I think they have been doing it to me. This person's career is bank security. Allegedly, he can tap into any phone and pull anything off it remotley without needing to use any malware or having physical access to the phone. What can i do to my iphone SE, if i can't block the eavesdropping, to just know when it is going on?

Added (1). he can also activate the cameras and microphones and gps track with the phone. I don't like this happening to me. How can i stop it?

People can tell you they can do anything. Your choice is to take them at their word or ignore them. Until they have proved to you that is what they have been doing with your phone, ignore it as BS. I have been using a cell phone since 1992, and so far all the "hackers" that have bragged to me about all the things they can do, have never been able to prove this with my phones or computers, no matter what information they have.

I don't think you can, ask your friend how he does it if you want to know