Here's a thought: Why not set up a free flight, take the aircraft up and save it. Then, go into the FLT file and see what the AcState is and paste that into your mission FLT.
But I agree. Normally, once you're airborne, ATC will switch over to center. However, if you have a flightplan filed, that may not be the case. If you file a flightplan but do not contact clearance delivery, ground, etc. that may cause confusion in the flight if you attempt to get ATC later. If you are tuned the tower and takeoff without clearance, you will of course eventually get the good old, "Blah blah blah, you're not cleared to takeoff. Flightplan cancelled. ATC services terminated. Squawk XXXX..." At that point, I believe the ATC menu should switch over to center.
Maybe try this too. If the first time you instruct a call to ATC is after takeoff, start the mission and leave the ATC menu open the whole time and watch to see if it changes at all. You should start with clearance delivery and ATIS as the first two options - or ground, if no flightplan filed. Once airborne, it should switch to tower and ATIS and finally, center and find airport (or whatever the menu option is - I can't think of it right now).