Hi Nikola, welcome to the forums.
Air Berlin is not on the list of callsigns that came with FS2004. Checkout this website to get the complete list of callsigns, aircraft types and models:
http://www.homepages.mcb.net/bones/04fs ... nloads.htmThe only way to get Air Berlin (and others) added to the list is to download and run a program called EditVoicePack (EVP). EVP can then be used to modify the "Voice Pack" of names that are built into FS2004. The EVP website should have a good set of instructions.
The first thing to do when you have EVP running is "Update the Voice Pack" from a pull-down menu. That will take you to the author's server and automatically download several hundred "new" names that have been created over the years.
Where to find EditVoicePack? If you can't find it here or at one of the other familiar sim sites, you can do a search and find it fairly easily. Or, someone else might follow-up this message with a more direct way of finding it.
Let us know if you have any success. If not, come back and we'll try to get it right next time.
Regards,
JerryH