At the end i changed the bose headset against a bose-modell without microfone and this works very well.

When you buy a headset with microfon for N900 (or in general for the N-series) you must pay attention how are configurated the 4 pins. In fact Bose uses the iPhone configuration, so they don't work with the most of Nokia phones.

Here you can see the difference of the pins.