Hello,
In the helicopter community in the Navy we used our squadron callsign for all our missions, except when we were on an IFR flight plan. In that case, we would use our AirWing tail letters, plus our side number (ex: NL 613, "November Lima 613").
In many cases you could tell a squadron by it's tactical callsign, but quite a few defied that logic. Here are some examples:
HSL-31 "Archangels" callsign: "Copyright"
HSL-35 "Magicians" callsign: "Magus"
One of the callsigns that the Air Force uses that always makes me laugh is, "Hefty". This is a Travis C-5 callsign, however, all dedicated AMC airlift missions use the callsign "Reach" no matter what type aircraft. At least that is the explantion I heard on that.
Regards,
ex-Navy Rotorhead
"Black Knight 613, your signal is Buster, and charlie spot three."