Earth shines like a star to the radio listener. Since 1919 we've been using that particular band of the electromagnetic spectrum for everything from TV and audio broadcasts to wireless networking. Over the last century the volume of our emissions has increased vastly, to the point where our radio signals - even the ones not intended to be heard by extraterrestrials - are audible over a sphere dozens of light-years in diameter.
One would not have imagined this would be a problem.
The first thing I learned from the aliens' reply message (reprinted below, with the dull preamble omitted) was that despite the fact that they're a few thousand years further along the technological development scale than we are, they haven't sussed interstellar travel yet. Now, either that means they're a bit slower on the technological uptake than we are (I mean, who knows if the technology explosion we've seen over the last seventy years happens to every species?), or space travel just can't be done. Which is kind of a lucky break for us, because the second thing I learned from their message is that they can hear radio signals.
Not using instruments. With their ears.
It would appear that they are somewhat irate about this.
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Hey, it could happen.