"Flight number"; "Callsign" -
But then the FPL refers to "Identification" - which is why the guy asked!
In case of any residual confusion:
Item 7
AIRCRAFT IDENTIFICATION (maximum 7 characters)
This consists of the aircraft registration letters or the company designator followed by the flight number which are to be used by air traffic services for radiotelephony communication and coordination.
This field can be filled in in one of the following ways:
a) aircraft registration when:
- in radiotelephony, the call sign to be used by the aircraft will consist of this identification alone (Example: DAGXC or N137DK)
- preceded by the ICAO telephony designator for the aircraft operating agency (Example: SMALL-AIRLINE DAGXC)
- the aircraft is not equipped with radio;
b) operating agency and flight number: ACA123, KLM672, BAW74W, etc.
c) tactical call sign (usually military): BRUNO12, SWIFT45, RED1, etc.
Flight Plan Filling - SKYbrary Aviation Safety
"Flight number" here means of course the ICAO Doc 8585 three-letter operating agency designator followed by either a numeric or alphanumeric (ie the ATC callsign, also the Mode S ACID) and not the one starting with the two-character IATA designator that's typically used on tickets and departure screens. ATC normally have no interest in the latter.