YEA NO KIDDIN i dream of hearing southwest in ATC, very sad...
Check this website if you want all the atc codes for airline callsigns...
Next, just go to the aircraft folder of the plane you want to change, (FS9/aircraft/b737_400/aircraft.cfg) and go to the aircraft.cfg document; open it with WORDPAD(right click,open with-> wordpad)!! Once you're there you will see where it says "atc_airline= " under the [flightsim.0] menu;now this is where you type in the airline callsign from the above website. Notice if you scroll down there are many other "atc_airlines= ",so you can change more names!!
i.e. type in "atc_airline=frontier flight" and atc will say"frontier flight 1123, cleared for takeoff". its gets ever more advanced and technical from here!!