Do you have a car which implements its own voice command feature? Chances are if its reasonably new you do. Which also means you're probably frustrated if you want to use your phone's voice assistant hands-free and wire-free over the built-in Bluetooth setup.
Personally, I like to leave my phone in my pocket when I'm driving. If I want to use Siri or Google Now that's impossible. The iPhone needs to be plugged in to allow 'Hey Siri' activation, which means wires trailing across my car. Ok Google works without being plugged in, but Google Now doesn't listen over Bluetooth, so its not even close to usable.
Windows Phone users however, have a completely hands-free, wire-free way of using Cortana in car, which appears to have been added to the last build pushed out by Microsoft. Click the voice command button on your steering wheel and say 'Call Cortana', which will then hand control of your phone to the voice assistant, using the microphone and speakers in the car.
A brilliantly simple solution to the problem.