Hello fellow pedant.
You have to extend the planes into the plane of the camera view.
When the front and back planes are extended it is entirely possible for the camera view plane to be perpendicular to both. In fact, it would be impossible for it to be perpendicular to only one since you pointed out that the front and back planes are parallel.
True. Mea culpa.
What I was objecting to was something else, implied in parent post, to the effect that the lens axis lay on the both the front and back planes of the phone at the same time. But I expressed it poorly.