It is a song from the time escorts in inner city flats you could call by phone became more common, tunneling under red light district regulations. This song is playing in Munich and the escort in question is a girl named Rosi.What does it mean?
Fun fact: While the band took care to check that the phone number they mentioned was not correct in Munich, it was an official city department number in a different city. They had a lot of "fun" there in the eighties with calls requesting a talk with Rosi.