How can I use Hey Siri with phone in pocket

I just got a 6s Plus and love it. But I was looking forward to using the new Hey Siri feature, using it even when the phone isn't charging, to make calls over the bluetooth in my car with the phone in my pocket while driving. But Hey Siri won't work with the phone in my pocket. Someone said that's because to the feature that prevents "pocket calls." if that's so, is there someway around it? It seems silly not to be able to use this feature to make voice activated calls while driving.