Regional airlines in the United States that fly under a major carrier brand still use the call sign of their airline. I wanted to use this forum to explain which callsigns go with the airlines and aircraft.
(Labeled Airline and Plane - Operating Airline/Callsign Airline)
Delta Embraer ERJ-170 - Republic Airlines
United Embraer ERJ-170 - Republic Airlines
Northwest Embraer ERJ-175 - Compass
US Airways Embraer ERJ-175 - Republic Airlines
United Express Bombardier CRJ-200 - SkyWest
United Express Bombardier Dash 8-Q400 - Republic Airlines
Many of the callsigns that go with the regional airlines in this game, actually do not operate the regional aircraft in the game. I hope this was a good and helpful resource!