if you want atc to call you by carrier and flight number you must edit aircraft cfg.

atc will fall back on tail number (reg.#) for call sign if not.

here's my united 757...same tail number as IRL, you can verify real tail numbers on the web...airliners.net or below

atc_id=N585UA
atc_airline=United
atc_flight_number=

I leave flight number blank so I can change it easier from UI details page, if you don't enter a flight number in the UI atc would call you "Boeing 5UA"

Make sure to have the callsign you want to use selected in the UI details page as well, most likely it will be there but it doesn't hurt to double check.

I like to mimic real world flights by searching AC tail number on a tracking site to see where the plane is at the moment, usually I find one of my planes on the ground and set up the flight like that using actual flight plan and number.   

Some have made lists of FSX ATC callsigns that work. I did a search and found a few on other forums. I can post the one I'm working on if you like.

check out this website if you haven't already

http://www.planespotters.net/

production lists of most boeing aircraft as well as others...tail numbers, status, etc.