I've been using cassette tape jack to listen to the music or handsfree in the car (for my old phone, Treo). After I change it to Curve, I expect the same function. Its sound is great as mp3 player. Now, if someone calls me when I listen to the music in the car, I expect it stops playing the music and turn to handsfree mode (which Treo did before). It, actually, stopped the music and connects the call. But I could not hear the voice over the car audio. Even though it is connected through 3.5mm audio jack, I had to hold it to my ear to talk/hear the voice.
I think it is something about setting (such as audio out through speaker or jack.... for handsfree) , but I couldn't find the right one.