Well, by default the device disables the FM Transmitter when you get a call - however, there's an app called FMTX Faker (though it's in Extras-Devel, not Extras, so use at your own risk) - it works by tricking the transmitter into thinking it's allowed to transmit, when normally some other program (like the phone) is telling it not to - so at the very least that covers the ability to receive a call and transmit it to your car at the same time, which is what the op was referring to, I believe...

To the last poster - if your FM earmuffs receive FM directly (and you're not talking about using headphones as the N900's FM antenna) then the above should cover it. You can get the FMTX Faker, and then have it set (I think it can be automatic) so that it tells the transmitter it is supposed to be enabled regardless of what else happens, when you get a call you can answer, enable the transmitter, and you're set.

There's another app, in Extras, which is called the "Simple FMTX desktop widget" - but I don't know if it can force the transmitter to be enabled in-call. What I would do is if you're not comfortable getting stuff from Extras-Devel, ask/read around, or download the Simple FMTX desktop widget first, then have someone call you and see if you can use the widget to enable the transmitter mid-call consistently. If you can, that's all you need. If not, but you really need the ability to have the N900 transmit the call by FM, then I suppose you could enable Extras-Devel, download the FMTX Faker, install that, and then disable Extras-Devel again (and it's probably safer to ignore any software updates while having extras-devel enabled because I THINK newer, less stable versions of programs are usually in the Extras-Testing/Devel repositories for at least a little while before they are good enough for end-user consumption).

- EDIT -

As for enabling full-on FM-Calling, which I can only imagine as both phones transmitting and receiving at the same frequencies (or Phone A transmits on the frequency phone B is picking up and vice versa, though that would be a pain to coordinate), without at all using their normal wireless networks, I suspect that something like that is possible, though you'd probably have it practically limited to walkie-talkie-like behavior. It could be possible to actually 'dial' another phone through FM radio, but I think that's ridiculously beyond the scope of practicality, since both phones would have to be actively transmitting some sort of identifier on the FM, which would honestly just cause FM jamming for everyone around you, and you'd probably have to come up with your own protocol just to make it work (maybe not - when it comes to the actual technicalities of programming I am a newb) - however, I suspect walkie-talkie functionality should be possible, if you could get the N900 transmitting and receiving on the same channel, without using the input from the FM receiver as output for the transmitter, since that would cause feedback. I haven't actually tested receiving on one channel and transmitting on another to see if that's how it would work, but I suspect that's exactly what would happen. ...if the transmitter of N900 A doesn't just completely over-power the signal of the other N900 B that N900 A's receiver is trying to pick up. That's more of a needs-to-be-tested hardware question, though.