I’ve been wondering, so I looked it up: An iPhone 6 has a focal length of 29mm*, which is considered wide angle, too wide to shoot natural looking head shots.
The article linked below shows what 28mm looks like, and says:
“85mm to 135mm is the focal range that’s commonly recommended for portraits, since it helps avoid the thin- and wide-looking distortions found with super wide or telephoto lenses.”
Cellphones are way TOO WIDE for natural looking, undistorted portraits — making the nose look too big, etc.. One remedy is to shoot further away, so faces look more naturally correct. But then they’re tiny in the image.
Companies are working at trying to make cellphone lenses that reach out further. In the meantime, at least one company makes lenses that screw into a cellphone case, which shoot in longer focal lengths. And many compact cameras zoom to 85mm.
* 35mm sensor size equivalent — the standard used in this example
PetaPixel shows in this article how facial features look photographed at different focal lengths: