Welcome to the Club IsonHaug.
The T-Point where I bought my iPhone also knew that I intended to use it outside of Germany and the salesperson confirmed that the unlocking would make that possible. That is the whole purpose of unlocking, is it not?
IMHO Unlocking is useful to people withing a country who, for a reason or another, absolutely want to stay with one carrier and, above all, it is useful to people who travel a lot and want to be able to use carriers from several countries to save on the roaming costs.
Today, a German buyer who gets an unlocked phone because he or she travels a lot and wants to use local operators in other countries doesn't get what they are promised.
T-Mobile Germany seems blissfully unaware of the problem. Apple Germany's Help-Line doesn't offer help. Apple Germany Headquarters doesn't answer to a letter I sent them by fax almost a week ago. I guess you can sense my mounting frustration...
You can file a bugreport at bugreport.apple.com (registration is required but you can choose one of the free options). The more people complain, the greater the likelyhood that it gets fixed, if only to stop those annoying complaints...
One more thing: As far as I know, T-Mobile does not allow you to return the phone for this problem. They're probably wrong in doing so, but then you will have to hire a German lawyer (or argue really well yourself).
While waiting for the solution, enjoy the chat here. An be aware of the posting rules in here 😉
Marc