Just following up on this -
Paddy - that's weird! Has it worked again or is working now?
Anerbe - do you have any other calling apps installed, like Skype? Was just googling around and wondered whether another app like this might interfere with it.
I noticed if I use the voice search feature in the Google app - this works over Bluetooth in the car. It's just Siri I'm having the problem with.