NASA and SETI astronomers picked up on a radio signal that seemed to have a coded message sometime in the mid 1980's. The signal was coming from Jupiter's moon Ganymede and was definitely artificial in nature. The signal was run through multiple computers and no sense could be made as to the meaning of it. President Reagan was notified and an international conference was assembled to determine how to make an attempt to contact whatever beings were sending the message. Eventually it was decided to send a signal to the exact location that the message was coming from on Ganymede. This message would be sent in Morse Code which was considered to be the easiest for an alien civilization to decode. It would read "We have received your signal, but we do not understand it. Please resend your signal using this language and transmission code." Immediately after this message was sent, the mysterious signal from Ganymede ceased its transmission so everyone assumed that we would receive a new signal in Morse Code in a quick period of time. But day after day and then week after week no signal came and officials began to give up hope of actually receiving a response.

Finally after more than a month, a signal was detected coming from the original radio source on Ganymede and incredibly it was in Morse Code. Eagerly NASA scientists translated our first direct contact with an alien civilization.

And it said "We were not talking to you."

Source

As to the signal itself, this is what Rupert Matthews reports in his book Alien Encounters: True Stories of Aliens, UFO's and other E.T. Phenomenon which was published in 2008.