Yesterday I called the Austrian LEGO callcenter (to renew my sons subcription to their club magazine). While I called an Austrian number, I ended up in Germany (easily recognisable by the agents accent). I gave them the necessary infos and endend the phone call by saying 'Auf Wiederhoeren'. Which is the proper formal way of ending a phone call with someone you do not really know. In Austria, at least.
The call center agent finished the German way, which is: 'Tschuuueehhhuuuees!' (more sung than said).
Now, 'Tschuess' is a rather informal way of ending a conversation in Austria. You only use it with friends. Yet it is totally accepted in Germany, even if talking to total strangers.
When I lived in Berlin some time ago, I was constantly feeling like a dork after saying 'Auf Wiedersehen/hoeren'. I felt very old-fashioned.. Yet I still perfere 'Auf Wiedersehen/hoeren' to 'Tschuess' (at least when speaking to strangers).
Oh, it happens the other way round, too. A friend of mine who is living in Berlin for several years once called his old professor at the Viennese university. He ended with 'Tschuuueehhhuueess!', heard his prof stutter 'Auf Wiederhoeren' (sounding slightly annoyed..) and thought 'Damn, he'll think I have no manners'