Rendering

Mostly you're rendering your file using some unsupported codec or file extension, be sure you use h264 so you can get an MP4  file that can work on iPhone 

As Mylenium says, we have no idea what the problem is unless you tell us the format, resolution and file size of your renders.

As a general rule, render an H264 mp4 file using Adobe Media Encoder and most mobile devices will have no issue playing the file.

How do you even transfer the file to your phone and where is it stored on there? What render settings? What else? You need to be much more specific!

Mylenium

You check the permission of the file and the folder in order to get access to the animation you wish to play.