Listening to music on the N900 is a good experience: system is smart enough to pause playback when a call comes in and restart it once it ends plus there is no need to don the headset since you already have it on.
No such luck with a secondary phone, for example the office mobile: all stopping/starting/changing must be handled manually by the user.
It would be nice to have the N900 act as a switchboard of sorts.
Talk thread here