IMO, how sad. We have had trouble at work with my boss saying we cant use our mobiles. I said to him, so do i hold you responsible if i miss an important family phone call?

That kinda put the tin hat on that discussion.

They can only detect the use of Cell Signals, and would be illegal for them to intercept text or other data to and from the phone.  It cannot be specific to detect if you are making a voice call or text because the signal is the same.  This may also be illegal without specific permits, it falls under FCC rules and regulations.  Jammers ARE illegal, but they are out there...

Also, yes, phones will not transmit when OFF, but when on and idle, they still do their cell pings.  They transmit and receive tower checks, if you wanna call it that.  It is like a ping, the phone pings a cell tower, then it responds, this is how they stay ready when not in use.