In his memoirs of his days as captain of Queen Elizabeth 2, Captain of the Queen, Captain Robert Arnott said that in 1978 the ship was near Tahiti when the radio room received "...
a signal prefaced by the letters GBTT - the call signal of the Queen
Mary." p. 233, Captain of the Queen, New English Library Ltd; 1982, ISBN 978-0450048913, and said that the signal had apparantly been sent during the Second World War and had been travelling in space for almost 40 year.
Maybe not. The first thing that I thought of was that the QE2 had the same call letters as the Queen Mary, GBTT, and that the radio room might have just gotten one of its own signals with a delay as it travelled around the world, which often happens to radio operators. Also, I'd imagine the Queen Mary would not have used its usual call signal during the war, since it was a troop ship and would have been a prime target. It would also have kept radio transmissions to as little as possible. And of course, 40 light years distance would make a ship's radio signal unable to be received.