I have Nokia ck whatever it is blutooth car kit, and iphone4 with tom-tom. All works very well thanks to recommendations here.
I had wondered if the voice commands from tom-tom would be transmitted via blu tooth and cut in by muting the radio etc as the phone function does nicely. Sadly not the case, and sort of accepted this wouldn't work, no biggy. Until...
I used the voice dialing feature on the phone. This also works well, press and hold menu button, voice comes in muting the radio "dialing Mrs Gixer" etc. All fine. But I later spoke a command that wasn't correct as written in the phone book, so got a voice saying "command not recognized" or something like, which repeated continually until I cancelled it via the phone. But within that time the tom-tom voice commands could also be heard in the background via the car speakers. Oooh!
Do I need a streaming car kit capable of streaming music to get this to work? I realize streaming music is pointless via a mono phone line in to the head unit but would this enable all sound from the phone to mute the head unit for Tomtom commands.... or whatever...? While the voice command feature is active on the phone Tomtom is not visible, so although the voice can be heard the map is not an aid.
Taking this forward, Siri would be interesting feature via head unit. Etc.
A member here mentioned parrots might be capable...?

Is it worth upgrading the car kit...?