@Roeland

I think the best value of use for i tool like FM CARKIT would be this:

1 No automatic activation or deactivation of the transmitter

2 If Carkit switched on: Calling sound and phone speker sound is transmitted. when Carkit is off: Normal state with no phone sound on the transmitter

The Advantage: Carkit is then only a Phone FM Transmitter enabler when the Transmitter is used in Car when Music runs. and could ever be switched on. When you're not in car the Transmitter stay off.

Peter from Munich