or the other altnernative way for now would be that you could connect a line in audio wire from the n900 to an fm transmitter an it will be able to play ur music as well as the calls on the radio. Or you could hook the n900 up to a cassette adapater that folk used to use for the portable cd players.
Again these are just ideas/solutions for now, until someone makes an app for the n900 to transmit the calls via its own fm transmitter.