Canberra:
Because, for example, 'Saxon' calling up and stating they're a bunch of tornadoes doesn't give away their Sqn and role - ok, so you probably know who 'mallard' are but that's the nature of their callsign - type specific, with a limited No of operators.
as for being the holy grail, it's not, but equally broadcasting it to all and sundry isn't a great idea....unless they're spotters !