Yes, there's some people here that thinks that not knowing the answer is a good reason to blame who questioned it. I saw some of them too.

Anyway, im a noob so don't take me too seriously, these are my two cents:
i personally dont think its impossible to transmit calls through fm transmitter. its just that someone has to write an app for it, cause at the moment is not made for that. consider also that a similar app should automatically increase mic volume in order to make it possible for you to speak at some distance from the phone and in a noisy environment as a tripping car is. anyway i think a big limit to this to be developed could be the poor quality of the fm transmitter itself (as of now).

sorry for my bad english and the casual bullshits i could have said.