I have a 2015 OB Limited and "Hey Siri" works for me. First, make sure that on your iPhone SETTINGS/SIRI/ALLOW "HEY SIRI" is on. Then use one of the USB ports to connect your iPhone via a Lightning cable. From what I have read, if the iPhone is not connected on 2015 OBs via USB cable, "Hey Siri" will not work. The 2018 OBs with Carplay should support "Hey Siri" simply by pressing the Voice Command button on the steering wheel.
I usually only use "Hey Siri" for map directions and for selecting music from my iPhone. But even then, Siri sometimes misunderstands what I want and I have to repeat my request.