It all depends on the type of signal which was initially received.
Hail: "Here we are! Is anybody out there? If you hear me please respond."
Warning: "Danger, stay away, no trespassing, git offen ma propty."
Monument: "We are the Krell. This is our legacy. Here is a list of all our accomplishments and shortcomings."
Eavesdrop: "...and that's today's stock market results...Now stay tuned for the Space Weather Report".
Direct Communication: "We know you're there and who you are. We need to talk."
Non-communication: Industrial by-product noise, military activity (hostile or not), navigational beacons, planetary defense and traffic control radars.
I could go on.