You can change the airline names to anything you want but there is a 'default' list of callsigns that you can't change unless you want to create your own by downloading and running the 'edit voice-pak' which I suspect you won't want to do.

There are some errors in  the standard MS voice-pak eg Voice 7 refers to China Southern as 'Cactus' which is incorrect.

I'm away from my own PC right now and I forget where, but the list of callsigns has been published several times on these forums. Also, you can find loads of voice-pak uploads that have been done for probably every airline you need - but you need to download the voice-pak editor to install them I think.

Probably Tipster will know!!  

Roger

Edit - BTW if you open up the aircraft.cfg file for editing with either Wordpad or Notepad you will see where the airline info appears in the first section of the file. Just change it to the airline you want (making sure it refers to the right texture/paint file) and the callsign will automatically change assuming it's on the 'default' list mentioned above.