Also some airlines tower callsigns are different from that of the flight number. I have noticed this with a few operators at the airport.
E.G TOM1234 for airport/pax may be TOM234V to tower.
I'm not sure if this is correct but i had been told it was to do with the Spanish (or some european) airports not being able to use 4 numbers in the same callsign? Therefore 3 numbers and 1 letter is used.
But all airlines tend to use a mix of what LSG has said above