I wouldn't be surprised if it just fell back to the iPhone itself when you didn't have AirPods. It'd be a pretty crap audio experience (both input and output) in comparison though.
It's difficult to gauge from the photos but I was thinking that the phone is seated too far from the mouth for reliable audio input (it certainly feels like I have to raise my voice, if not shout, whenever I'm talking to someone with the phone that far away from me).
