A related issue, but a bit of a tangent from the original question: Here in the western US, it's common for
the center controllers to use multiple transmitters for a given frequency and switch between them as
necessary. They will activate the transmitter closest to your position only when talking to you. This can
be confusing as you monitor the frequency and realize you're not hearing the controller all the time, but
you do hear other aircraft responses (for example if they're at a higher altitude). When you first
experience this, you're sure they've forgotten about you or you missed a handoff. I even recall one flight
where there were so many aircraft getting confused by this that the controller made a broadcast
announcement to explain the situation, "don't be alarmed if you don't hear all my transmissions" or words
to that effect.

Mike