Part of a trial I once had to do on a military aeroplane was to transmit on both HF sets on nominated frequencies to check that there was no RF interference to the New Toy which had been installed...... The beardy-weirdy boffins would watch for stray wiggly amps upsetting the Thing.
All went fine - no RFI. But I did start to wonder a bit about the frequencies though, as music was playing on one of them. Finally, did the last check "One two three four five, once I caught a fish alive. Six seven eight nine ten, then I let the bug ger go again! Testing, testing, this is the voice of the Mysterons!!" To which an outraged voice immediately replied "WHO IS CALLING ***** RADIO??".
It seems that the mad scientists who had set up the trial hadn't bothered to check whether the frequencies they'd allocated were free to be used!
When BBC World Service does its Lilibulero theme tune and "This is the BBC World Service", there's just enough time to add "Hello, World!". Allegedly.
I did know of one wag who used to tape SelCal tones, then play them back on the relevant "Company" frequencies..... "Birdseed Ops, this is Birdseed (blah)" "Go ahead" "You called us!" "No we didn't!" "Yes you did, the SelCal just chimed" "NO WE DID NOT!!"
Very childish,of course.