Some crew would stick a note on the forward instrument panel with the flight number in large letters for all crew to see, viz. Speedbird 509. I was amused one day to see Birdseed 509 posted ! Don't recall anyone actually transmitting that.
An earlier post queried the call sign Clipper One, or Two. and a response was that Clipper Two was the Round the World service Westwards from USA. Not sure if all airlines followed the protocol, i.e. was it an ICAO thing ? but I recall flight numbers followed the principle " Evens go East " e.g. BA 510 went to New York and BA 509 came back.