heh, sorry for the delayed response (out all day etc) i had tried to connect through the transmitter prior to asking the question. i know that you can use the speakerphone but lets face it, another use for the fm transmitter and using the radio to call in any car would be quite cool apart from the obvious security risks. was just trying to see how viable it would be :P

@ That One Guy - the more effort you put into trying to make other people look stupid by jumping to (incorrect) conclusions and then attempting to berate or belittle only serves to highlight your own idiocy. You're what's wrong with the internet. Shut up you flid.

I am sorry for brining up an old thread but I just wanted to say that a few days ago I was listening to internet radio using my car stereo via the FM transmitter, when I suddenly received a call, I answered it, the N900 hang for a second and I could hear the call via the car radio (instead of the standard beeping sound).
(I could switch to speakphone and the phone speakers would work, and when I turned the speaker off I could continue talking via the car stereo).
anyway, after that call I tried to initiate another call but this time it didnt work, the call would come from the phone.
too bad this "bug" happend just one time and I dont know how to make it happen again.
But this proves that its technically possible.

If anyone could tell me how such a program could be written (what libraries etc.) I would be glad to try it.

Did the other person hear you when you were talking?(conversation coming through car speakers) Or you had to put mic near you to speak back..?

Well, I held the phone next to my ear when I started to talk but then I held it a bit farther, and the other side heared me with no problem (at least I think so, he didnt say "What?" or asked me to talk louder)

Anyone working on this or is it even possible? My workplace is pretty loud so recieving calls through my fm-earmuffs would be great! Unfortnuatly I'm totally clueless on the programming front.

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.

FMTX Faker tricks the FMTx daemon into thinking it can be enabled initially; devices with it disabled will show a "FM transmitter disabled" message regardless of the situation you're trying to utilize it in.

The Simple FMTX Widget just turns on transmission through the D-Bus interface provided by the FMTx daemon - fmtxd can and does veto the turning on of the transmitter in certain cases.

Ah. My apologies for the mis-information then. So, in that case, if you don't mind explaining, I have to ask - what practical application does the FMTX Faker have?

I believe people in Serbia, Greece and Hong Kong have use for it as the regulations of those countries required Nokia to disable the transmitter in software in devices sold in those countries

(Although it's said that 5800s sold in those countries have their transmitters working regardless...)