Easy! Just edit the aircraft.cfg file. Here's my 'standard' posting on this.
Every FS2002/4 aircraft has an aircraft.cfg file and there are two sections that you need to look at. Look at the following examples that are taken from one of my POSKY 767s.
Firstly the [fsim.n] section. If you only have one aircraft in the folder (ie one texture folder) this will be fsim.0. If you have several (like the various default 737s, you will have [fsim.0], [fsim.1] etc. Each will have section similar to the following.
[fltsim.0]
title=British Airways 767-336ER - China
sim=767300ER
model=
panel=
sound=
texture=
checklists=
atc_airline=Speed bird
atc_id=G-BNWC
atc_flight_number=1969
atc_heavy=1
ui_manufacturer=Boeing
ui_type=767-336ER
ui_variation=British Airways 'China' Tailfin Livery
visual_damage=1
description=Project Opensky Boeing 767-300ER\n....etc blah..blah
To get proper ATC for BA you need a bit like the part shown in red.
The part shown in green incidentally controls how the plane is shown in your list of aircraft.
Now the [general] section. This applies to all aircraft in the folder however many there are. It will look something like this.
[General]
performance=Passengers\n3-class conf...etc..blah..blah
atc_type=BOEING
atc_model=B763
You must have two lines like the ones in blue above for correct ATC plus you can't just type in anything - only certain descriptions are recognised. Someone listed them somewhere eg you can't get Concorde to come up, only Britishaerospace. (Actually Concorde is available and I use it).
Try experimenting. If you leave flight number blank ATC will revert to tail number so 'Speed Bird 1969' will become 'Speed Bird Golf minus Bravo November Whisky Charlie' the minus is pathetic I think - I don't know why M$ couldn't have corrected it in the final release and leaving it out in the atc_id part gives false call outs.
If you leave airline out assuming the type in general is correct, ATC will use type and tail number.
Hope that helps.