Different airlines have different formats for their numbering and callsign formats.
Sometimes they match, but usually they differ.
For an easyJet example, Gatwick to Gibraltar is EZY8901, callsign Easy 61HC.
This number and callsign pairing is the same each day for the early morning flight to GIB.
If conducting a ‘ferry’ flight, where an aircraft is flown without passengers or cabin crew, ot will be numbered and callsigned EZY9001 and upwards.
G (Great Britain) registered aircraft are callsigned Easy, whereas Austrian reg use “Alpine”.
I hope this helps,
David
FS ACADEMY
Extra: Also, now that Gatwick South Terminal has reopened, easyJet flights will have flight numbers starting with 6, to designate the use of the South Terminal.