Apart from the obvious, such as what has to be read back, callsign use, general RT practical use etc, I have another little thing that I learnt during one of my first line checks during commercial aviation: "every number has a name".
Trying sound cool, like all those other pilots, I'd say something like "Callsign XXX, 159 for 240 to YYYYY". The check pilots logic was that since "every number has a name", it should be "Callsign XXX, flightlevel 159 climbing flightlevel 240 inbound YYYYY".
Obviously, since my callsign that day was "xxx 61" it would sound like "xxx 6115942402yyyyy", which makes a whole lot of sense to me, since I said it, but it might not to others.
I've never seen said "rule" in any book, but I think it makes good sense.