Yes, it is. There is nothing that says that a phone can't show you the normal menu while being in a conversation with somebody. Get the picture? I know for a fact this can be done.

So, if you want to make sure the phone is off, remove the battery.

Even if there is, the vast majority of phone calls passes through government controlled telephone exchanges, so it wouldn't matter.

But he implies that you can be listened even when you are not talking on the phone.

Yes - and he is right. The phone is in permanent connection with the network anyway - it's some code away from opening the audio to listen to your room conversation.

My name begins with the letter A, so I get quite a few accidental calls when someone leaves their phone unlocked in their pocket.

Mostly you just hear an incomprehensible garbled rustling noise.

Spying on someone by remotely turning on the phone in their pocket isn't usually going to work.

Sometimes it's not. But if you're determined enough to spy on somebody via the audio on their phone, you will likely be persistent and eventually overhear something the person would prefer you didn't. If I were concerned about being spied on I wouldn't dismiss it so easily.

Although you are correct about the pocket thing, the point is that you don't know when someone may be listening to you.