Not only a Camera needs a ND filter, iPhone and smartphones also need one.
We have the lens fits for all the smartphones. including iphone and even iPad and other tablets.
as we are using such a clip:
to screw with this lens:
and it looks like this one:
When it is use with iPhone or smartphones, it looks like these:
Ok, you may not like to use with a clip, it is universal, but you know, it is not perfectly compatible, so you we have custom cover to make it perfect: