Thanks everyone. i think i understand now what the phrase "call waiting" in english means. of course we have this too - at least i can choose to activate hearing a signal when i'm in a call myself and somebody else calls me.

i'd bet, though, that this calling person wouldn't be told that his call is waiting. he'd hear the normal signal and would assume i didn't hear the phone if i don't answer after a while. on the other hand - nobody i know (including myself) has call waiting activated, so that could have changed and i'd never know.