Which is why I snort in derision when on a TV show they still have to get the bad guy to stay on the line for a minute. Can't they just look at the Caller ID? It makes me nuts to know that I can tell who's calling before I answer the phone but the cops have to figure out to keep the guy on the phone for a minute.
From what I’ve read, it’s possible to turn your phone on remotely and use it as a listening device. It’s definitely possible if the phone is already on. And, it’s legal - a judge issued a wire tap warrant back in 2006.
http://www.zdnet.com/news/fbi-taps-cell-phone-mic-as-eavesdropping-tool/150467
I think you can avert this by taking out the battery, but I wouldn’t swear to it.
Sometimes it’s actually comforting to me that I can’t get a decent signal inside my house!
Caller ID can be spoofed. Think of the number showing up on your handset as something from Wikipedia - it could well be true.