To get some of them to work. Open FSX. Go to Free Flight and change the current aircraft. HIGHLIGHT the Captain Sim 767 livery you want and then select DETAILS. In the Airline call sign: select the one you want. Then change Flight number: to the number you want.

e.g. Select 767-300 British Airways. Then details. Then in Airline Call Sign, select Speed bird from the list. Then in Flight number, type 763. The ones listed work.

I actually edited my aircraft.cfg and added the information into there. Here is the listing for my 737-300 RR British Airways:

[fltsim.5]
title=CS 767-300 Captain Paint5
sim=B767-300RR
model=rr5
panel=
sound=RR
texture=british
kb_checklists=
kb_reference=
atc_id=G-BNWW
atc_airline=speed bird
atc_flight_number=763
atc_heavy=1
ui_manufacturer="Boeing"
ui_type="767-300 RR"
ui_variation="British Airways"
ui_typerole="Commercial Airliner"
ui_createdby="Captain Sim"
description="767-300 Base Pack v1.4\n(c) 2009 Captain Sim www.captainsim.com"
ui_wglts=1

Here is the original:

[fltsim.5]
title=CS 767-300 Captain Paint5
sim=B767-300RR
model=rr5
panel=
sound=RR
texture=british
kb_checklists=
kb_reference=
atc_id=N764A
atc_airline=
atc_flight_number=
ui_manufacturer="Boeing"
ui_type="767-300 RR"
ui_variation="British Airways"
ui_typerole="Commercial Airliner"
ui_createdby="Captain Sim"
description="767-300 Base Pack v1.4\n(c) 2009 Captain Sim www.captainsim.com"
ui_wglts=1

I have tried adding more without success. I believe that you need to modify something inn FSX to get more of the callsigns to be stated. I tried modifying the airlines.cfg by adding them. It didn't work. I went online and looked up the call signs for the airlines and tried adding those instead. It still didn't work.

In FS9 you needed to add them to the ATC voice commands using the SDK. I will look into how it needs to be done for FSX. I stopped trying after those attempts at doing it (as you can probably tell) and will look into it again.

Hope this helps.

Mark