It is my understanding that some entertainment venues are places where
the use of a cell phone is verbotten.  This understanding also includes
the venue has posted this restriction in some manner as to be
informative for all attendees, likely on the admission ticket.  These
venues are often held on private property, so the legality of doing so
is a non-issue.

Yes, they use an electronic jamming signal with limited range, intended
to only cover the venue itself and very little beyond it's perimeter.
The jammer can be easily switched on and off at will, and yes, it blocks
all calls, incoming and outgoing.  Inbound calls won't complete and
simply divert to a voice messaging system, assuming one is available.

As far as emergency calls are concerned, that's what the venue staff is
for.  They have other means of communication that isn't jammed.

Frankly, given the disruptive and intrusive nature of cell phone use, I
don't blame them for doing this.